Allow Dash to be opened from the "Quick Help" popup without replacing the popup entirely.

Fixes #14.

The "Enable Dash Quick Help" menu item has been replaced by three options: disable Dash altogether, 
replace the Quick Help popup, or replace the documentation viewer. The default option is, as before, 
to replace the Quick Help popup.

When the third, "Replace Reference" option is selected, the plugin allows the Quick Help popup 
to be shown, but then intercepts taps on links in that popup and shows Dash instead of Xcode's 
documentation viewer. This works both for "reference" links (at the bottom of the quick help popup)
and other links.