Open all doc links in Dash.

This adds the ability to intercept option-double-clicks on symbols,
as well as to open "quick help autocomplete" links.

This feature can be disabled, like the rest of Dash, by selecting the "Disabled"
menu item within the "Dash Integration" submenu of Xcode's "Edit" menu.

I didn't add an option to specifically disable opening doc links (while still replacing quick help
or the reference viewer when opened from quick help) because I figured that anyone who's
using this plugin must prefer Dash to Xcode's doc viewer (while their feelings on the Quick
Help popup might vary).