Simplify, tidy, and improve gitignore.py

- Read list of available boilerplates from subdirectory
  - Easier to maintain

- Don't inherit sublime.Window
  - This is a WindowCommand, not a Window

- Removed duplicate code
  - first_select, second_select
  - 4 occurrences of essentially the same list

- Removed hard-coded paths
Remove .pyc file

Hmm... this repo needs a .gitignore! :-P