Make diagnostics display configurable

[tomv564]

In other words:

• allow turning off the panel
• make sure regions are visible and have hovers, perhaps status message?
• make phantoms configurable

[tomv564]

Added auto_show_diagnostics_panel setting in 797b373

Added show_ diagnostics_phantoms setting in 81e17b5

[rwols]

perhaps status message?

Yes, I really like this feature from SublimeLinter, consider implementing that too.