Packages submodule, git files, and docs improvements

[jrappen]

Update changelog:

• added changelog info from releases to changelog file

Changes to Packages submodule target:

• change submodule target Packages temporarily to st3 branch
  • this should be reverted to targetting master once sublime-syntax branch_point support #271 is implemented
• address Update packages #246
• address Update submodules #166 (which superseded Update submodules #154)

[trishume]

Thanks! I like these changes, except for the CSS colors.

The CSS colors are clearly generated but the generation code isn't included and it's just pasted into a file that has other non-generated code. Also if the CSS colors are going to be used they'll need to be in some kind of map indexed by strings and not just Rust constants. I'd prefer you just remove the CSS color stuff from this PR, and if you plan on implementing sublime-color-scheme support you can include it there and ideally in a separate file containing only generated code and checking in the script that generated that file. Or use a different approach like the default syntax packs use where you load a binary dump of a string to color map that's generated by a program under examples.

Otherwise I like these changes, especially the much improved changelog and the new .gitattributes. If you remove the CSS color stuff I'm happy to merge this.

[trishume]

Oh also I think in order to get the Packages actually updated you have to do some check out step in the submodule. I forget exactly how submodules work but I'm pretty sure this PR doesn't actually update the packages, just makes future updates target the correct branch. Also when really updating the packages you need to rebuild the checked in binary dumps.

Not updating the packages in this PR is fine though. I'm pretty sure based on the other issues that updating the packages might not work.

[jrappen]

Can't test the build on this machine. Might try later today.

Pushed the requested changes regarding css names for now.