Modify markdowntowp regex to recognize ** or __ for bold

Extended markdown processing of plugin meta to handle both **Contributors** and __Contributors__ syntaxes (I find the latter more readable, as in [this readme](https://raw.github.com/acusti/simple-footnotes-editor-button/master/readme.md))
Merge pull request #4 from acusti/patch-1

Modify markdowntowp regex to recognize ** or __ for bold