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.

[dacap]

Hi @zimoun, I'll take a look to this in these days.

[dacap]

Should load-previous-stats be defined in a (let...) block?

[zimoun]

Added.

However, there is still two issues:

• the first does not come from the keyfreq-merge feature. I am not able to run emacs -q -l keyfreq.el and I do not understand why. Maybe, I will open a special issue about that.
• the second is more annoying: if keyfreq-mode is turn on, then the resulting file-C will contain the stats from the file-A and file-B, and also the stats of the current session. Which is unexepected. I am not sure to understand why...

I am working on it, to fix that. Otherwise, it does not make sense. Sorry for this partial PR.

[zimoun]

My bad, sorry !

The second issue is know fixed.