Open documentation feature

I realised just now that I wrongly made a pull request from my `rob` branch into your `master` branch, so I made a new pull request from my `develop` into your `develop`.

This PR contains code refactoring based on your feedback:
- Move the keyboard shortcuts from the OSX into default mapping file
- Replace the `try/catch` blocks with conditional statements and sanity checks
- Remove the debugger print lines

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Let the user open their documentation straight away from Sublime by using contextual menu or keyboard shortcuts. The user can configure the package globally using the default and user settings, or on a per project basis using sublime-project files (Sublime 3 only).

Feature commands:
- Open live documentation index in web browser
- Open live documentation of currently edited in web browser
- Open local documentation index in web browser
- Open local documentation of currently edited file in web browser

Settings used by the feature:
`jsdoc_live_docs_url` - The URL of the live project documentation's build folder
`jsdoc_local_docs_url` - The path to the local project documentation's build folder