Support user-defined keywords

[bstrand]

As a Glyphfinder user, I want to add my own, custom keyword/tags for a glyph to the index, so I can remember it more easily or type it faster.

It would be shiny to be able to add my own tags to a glyph in the app UI.

Also / alternatively, being able to maintain one's own set of keywords in an external file would be useful for persisting across installs or for sharing. This would also enable users to create keyword lists in other languages.

Extracted from this comment: #25 (comment)