Find a way to allow merging of user aliases

[dantman]

Currently Sublime only allows user settings to entirely overwrite the default settings.

This means that if the user wants to specify a few aliases they will remove all the helpful defaults. Which is typically not what they want.

They could copy it, but then it would become out of sync as we add new ones.

We should find a way to allow user aliases to overwrite individual aliases instead of the entire aliases object.