Add the ability to ignore weeds without creating roots

[ocharles]

I currently have a module that runs some functions for benchmarking purposes. I don't want the existence of this benchmarking to cause code to always stay alive, so I can't add the module to roots. However, if I don't do this, then the whole module appears unused, which is also not what I want! I think we need a new option here, to specify weeds that should be ignored, but shouldn't become roots.

[ocharles]

The workaround at the moment is to delete any corresponding .hie files. This doesn't give us much granularity though - it's pretty all-or-nothing.