Integrate better with YASnippet

[piknik]

I found the file-based regexp to be too limiting. In my case I needed to specify specific paths but forward-slash characters cannot be used in filenames.

So rather than come up with a new system, we'll just use yasnippet itself to specify the regexp. As a bonus, we can have regexps be auto-generated. For usage see documentation additions.

Added code to scan yasnippet tables and use snippets from that system. yatemplate will auto-reload along with yasnippet. Documentation is modified and added to reflect new change. Previous file-based mechanism unchanged for backwards compatibility.