Allow note text and background color changes.

[jeff-h]

As per #63 this PR adds the ability to select the colours used for the note text and the note background.

There are a few issues, hopefully minor:

• I had to add two colour wells in the prefs for this, rather than using the existing ones provided by the NSFontPanel. Nothing I tried could get the colour info out of the NSFontPanel :(
• I couldn't turn off the colour wells inside the NSFontPanel — the commented code in my PR does remove them, but the method signature changed in 10.13, apparently, and thus won't work for earlier targets.
• I haven't applied the selected colours to the markdown preview. I am not sure if you'd want this (I don't use markdown myself). To achieve this, I guess you'd have to inject a line of CSS into the WKWebView setting the font colour and page colour?

[greendog99]

Love this PR! However, if you set a dark background, the cursor (which is always black) becomes impossible to see. I suspect a good solution is to set the cursor color to be the same as the foreground color?

[jeff-h]

The precedent set by Apple in TextEdit is that the text cursor is always black regardless of the text background. I found that a little surprising at it's unusable!

My new PR turns the cursor white when the background passes 50% darkness. I think you have to be pretty careful with colour on UI elements or the app will look amateurish so I hope you agree the white works?

[greendog99]

Looks like nvALT uses #474747 for the cursor when the background is dark. Not saying that's the right way to do it, just another comparison. I'm fine with #FFFFFF. I just like being able to see my cursor :-)