Allow use of function references as callbacks

[HiPhish]

Description of Changes

Closes #1066

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New Version Info

Allow use of function references as callbacks instead of only strings in menu API. This approach could be extended in the future to support function references for all callbacks.

Author's Instructions

• Derive a new MAJOR.MINOR.PATCH version number. Increment the:
  • MAJOR version when you make incompatible API changes
  • MINOR version when you add functionality in a backwards-compatible manner
  • PATCH version when you make backwards-compatible bug fixes
• Update CHANGELOG.md, following the established pattern.

Collaborator's Instructions

• Review CHANGELOG.md, suggesting a different version number if necessary.
• After merge, tag the merge commit, e.g. git tag -a 3.1.4 -m "v3.1.4" && git push origin --tags

[PhilRunninger]

I like this. While you're at it could you similarly update the NERDTreeAddKeyMap() API function? The function that you'll have to change is: https://github.com/scrooloose/nerdtree/blob/master/lib/nerdtree/key_map.vim#L66-L75

[HiPhish]

@PhilRunninger Done, I also did it for paths (https://github.com/scrooloose/nerdtree/blob/master/lib/nerdtree/path.vim#L503). Let me know if there are any more places to patch. I'm feeling under the weather right now and I'm not in the mood to go digging through the code, but if one can tell me approximately what to look for I can do it.

[PhilRunninger]

Looks like there's one more here: https://github.com/scrooloose/nerdtree/blob/master/lib/nerdtree/notifier.vim#L18

Also , make sure you update the CHANGELOG.md. Use version number 6.4.0, since this adds some new functionality.

[HiPhish]

@PhilRunninger Done.

[PhilRunninger]

Just a couple small changes left.

1. I ran through a few tests of the code to check for errors. I found a couple places where a variable needs to be made uppercase.
2. I like the compactness of the ternary operator, and would like to see it throughout your code changes.
3. A minor formatting change to the change log.

[PhilRunninger]

Suggested change

	- .0: Allow use of function references as callbacks (HiPhish) [#1067](https://github.com/scrooloose/nerdtree/pull/1067)
	- **.0**: Allow use of function references as callbacks (HiPhish) [#1067](https://github.com/scrooloose/nerdtree/pull/1067)

[PhilRunninger]

Replace this if block with the ternary operator assignment below.

    let Callback = type(self.callback) == v:t_func ? self.callback : function(self.callback)

[PhilRunninger]

Replace this if block with the ternary operator assignment below.

        return type(self.isActiveCallback) == v:t_func ? self.isActiveCallback() : {self.isActiveCallback}()

[PhilRunninger]

listener needs to be capitalized here and in line 17. Replace the if block with this ternary operator assignment.

    for Listener in s:Notifier.GetListenersForEvent(a:event)
        let Callback = type(Listener) == v:t_func ? Listener : function(Listener)

[PhilRunninger]

callback needs to be capitalized here and in line 503.

        for Callback in g:NERDTree.PathFilters()
            let Callback = type(Callback) == v:t_func ? Callback : function(Callback)

[HiPhish]

@PhilRunninger Done.

I like the compactness of the ternary operator, and would like to see it throughout your code changes.

I like it as well, but I thought that the lines would end up being too long, so I had opted for if-blocks instead. I have changed it to the ternary operator now.

A minor formatting change to the change log.

I think I should turn off concealing for Markdown buffers, at least if the buffer is not nofile. I was wondering why none of the other changelog entries had ** either.