Custom Search Prefixes for installed docsets

[gozes]

Currently, there is no way to change the search prefix for a docset which is fine because the docset feeds that come with zeal have sensible search prefixes. However, that doesn't work so well when you start adding other feeds. for example, if you add the aws cloud formation feed from https://github.com/hashhar/dash-contrib-docset-feeds you will notice that the search prefix for it ends up being AWSCloudFormationTemplateReferance which is quite long for a search prefix. I would much rather be able to change the search prefix to something like cf than have that long prefix.

Yes I know zeal can tab complete but I find it easier to type something like cf:<search term> then type a few letters, make sure zeal has selected the correct suggestion, hit tab, then and only then can I start typing the search term

[trollixx]

Should be possible to do, but currently all metadata is stored along with the docset itself, so it wouldn't survive updates. Will need to address that first.

Somewhat related to #77.