refactor
run Build.ps1 when pressing F7 (if 'Run' build system is selected)

The build system will fail because it can't find the packages
path location. This fill be fixed later.
edit build.ps1 to optionally use a config file

- ignore said config file (plugin_dev.json)
- wire up build.ps1 to .sublime-project file

Instead of having to define $STPackagesPath in every session,
it's now possible to store that value in a 'plugin_dev.json' file.
This file lives in .../sublimetext. It's a json object that must
contain a `packages_path` key.

This file is ignored in .gitignore, as it only should be available
locally.

Build.ps1 is now wired up to .sublime-project as the default 'Run'
action. This way, it's possible to press F7 to deploy the files
from within Sublime Text.