Proposal: Change how settings are handled

[ehuss]

The current way the settings are configured cause a few problems. The crux of the issue is that there is a Rust.sublime-settings file in the package. The problems with this are:

• Due to the order that settings are loaded, this overrides any ability to change these settings per-project (in .sublime-project).
• The syntax checker clears any highlighting whenever you save a non-rust file because the syntax-specific settings are not in scope for non-.rs files.

I'd be happy to submit a PR if you would agree to changing this. The changes are very simple:

• Remove Rust.sublime-settings from the package.
• Add defaults whenever settings.get(...) is called.

I would also like to change the "projects" setting. I think it should be changed so that target overrides could be placed directly in a .sublime-project file. Also, the name should probably include the word "rust" since this is a global namespace.