Allow Dash to be opened from the "Quick Help" popup without replacing the popup
entirely. (refs #14)

The "Enable Dash Quick Help" menu item has been replaced by a "Style" menu
which gives 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, using
the last path component of the link URL as the string for which to search.
This works both for "reference" links (at the bottom of the quick help popup)
and other links.
Properly determine the search string for links to release notes and technical notes. (fixes #14)
Open quick help reference links as Dash URLs.

Rather than merely searching for a term extracted from the link information,
we now open the link directly in Dash, which allows us to disambiguate
between similar results and even return a result if a simple text search were to fail.

We open the link by preparing a URL according to the `dash-advanced://`
scheme, consisting of:

    1. The name of the result/page to show (e.g. NSAlert)
    2. The type of the result (e.g. Class)
    3. The path to open (e.g. the docset URL)

Per @kapeli, given this URL, Dash will then:

    1. Perform a regular search for the result using the currently enabled docsets
    2. Search for a result that matches the path
    3. If a result is found, that result will be prioritised and it will appear as the top result
    4. If a result with that path is not found, a fake result will be added to the top
Refactor Dash's menu to better surface integration options.

Per a suggestion from @kapeli.
Describe the "Replace Reference" option under the "Usage & Installation" section of the README.

For clarity.