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
Merge pull request #195 from my-personal-forks/format

Add basic support for `dartfmt`