add helpers

Helper code copied from a similar plugin for the Dart language.
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Your branch is up-to-date with 'origin/master'.
#
# Changes to be committed:
#	new file:   .gitattributes
#	new file:   sublime_plugin_lib/__init__.py
#	new file:   sublime_plugin_lib/collections.py
#	new file:   sublime_plugin_lib/filter.py
#	new file:   sublime_plugin_lib/fs_completion.py
#	new file:   sublime_plugin_lib/io.py
#	new file:   sublime_plugin_lib/panels.py
#	new file:   sublime_plugin_lib/path.py
#	new file:   sublime_plugin_lib/plat.py
#	new file:   sublime_plugin_lib/sublime.py
#	new file:   sublime_plugin_lib/subprocess.py
#	new file:   sublime_plugin_lib/text.py
#
Merge pull request #795 from fsharp/new-sublime-helpers

add helpers for Sublime Text plugin