configure enabled via lua function for individual sources

[jmbuhr]

First of all, thank you for this amazing plugin and making it so easy to extend!

I wrote a plugin that comes with a nvim-cmp source to provide code completion for embedded languages (think code chunks in markdown documents): https://github.com/jmbuhr/otter.nvim.
Right now I deliver these code suggestions everywhere in the document, but it would be handy to only do so when inside of one of said code chunks. It may be an elegant solution for this and other usecases to add the enabled option as described in the wiki on the level of individual sources in addition to the overall setup. What do you think?

[jmbuhr]

On second thought, for my particular usecase it was the better solution to build this straight into the is_available method of the completion source (jmbuhr/otter.nvim#6).