Log level

This includes two more significant changes and may need more testing.

1. Tag as case class with two fields: tag and level, the latter of type sbt.Level.Value. This allows each tag to appear as warn, info, error, or debug, and have warn as a default. A map might be simpler, but loses the default ability.

2. Only match tags in comments - not sure if this is a necessary feature. The two commits might be a little muddled and need pruning. It could also be activated and the syntax could be set through settings. For now, it assumes single line //, and multiline /**/. The parsing could also probably be done more simply with parser combinators or with the help of regexes.