#24 New Frequency List functionality

[Shane]

New functionality to support tasks identified in methodology papers.

-Manual term selection and thresholding of frequency lists
-Saveing of term lists for reuse in other plugins

Terms could be used for a concordance search, could be hilighted in a Mosaic orcompared across corpora (comfre).

Lists of terms could be used to represnt concepts. Automated methods of extraction could be explored.