Add ability to filter search by docstring

Adds a little syntax to allow filtering the search results to a specific (or a set of specific) docsets. This is a lightweight implementation of the similar feature in Dash.

The docset filter and the search query are delimited by a pipe character (`|`).

Examples:

* `ruby|set_x` will search all docsets containing `ruby` (e.g. `ironruby`, `ruby` and `ruby2`) for `set_x`

* `set_x` will search all docsets for `set_x`

* `||set_x` will search all docsets for `|set_x`.