Add key binding

- Limit key bindings to command_mode == false

This should improve cooperation with other plugins (for example,
Vim emulation.)

- Add `Ctrl+., Ctrl+.`.

Shows all 'Build: Dart:*' items in the command palette.
Merge pull request #192 from my-personal-forks/master

Add compatibility matrix to README
Add utility class to filter text through external program
Add basic `dartfmt` integration

- Adds ability to run selected text in view through `dartfmt`.
- Hooks functionality up to build systems.
Add ability to reformat whole file using `dartfmt`

- The content of the file is passed as a string to `dartfmt`.
  This way, the user should have a chance to undo changes
  within Sublime Text.
Minor refactor
Fix #186 - Add message for first installation
Merge pull request #196 from my-personal-forks/install-message

Fix #186 - Add message for first installation
Merge pull request #195 from my-personal-forks/format

Add basic support for `dartfmt`
Merge pull request #179 from my-personal-forks/keybindings

Add key binding