Adds Dart support

[Levi-Lesches]

My solution to #1 (it's not a generalized solution, I just figured out how to add Dart).

Note: I added a field project to CoverMeModes.sublime-settings, because some languages/environments need the test command to be run in the root of the project, but currently basepath is set to use the path of the file instead. I don't know how that affects Python and Go, so I just made the project field optional and only used it on Dart.

As an example: if you have this file structure:

my-coding-folder
|- my-project
   |- tests
      |- test_1.dart
      |- test_2.dart
   |- lib
      |- main.dart
      |- data.dart

Currently, when running CoverMe on man.dart, basepath was set to my-coding-folder/my-project/lib/main.dart, but Dart really wants the command to be run in my-coding-folder/my-project.

Of course, every language has their own idea of what is a "package", so instead of using Dart-specific code, I simply check which folder in the Sublime project contains the path of the current file. In my case, I'd have my-coding-folder/my-project added to Sublime, so that would be chosen as the project folder. And of course, if someone uses the basepath option in their .sublime-project file, they would have complete control over this manually.