change default values and add keyfreq-merge feature

[zimoun]

The first commit fixes #7. The second commit fixes #12, at least, it adds a new function able to merge two stats' files, following my proposal in the disucssion in #12.

It is just some tweaks around the existing functions keyfreq-table-load and keyfreq-table-save. It should work on the fly, using the already in-place lock mechanism.