Add: beta parameter for F-measure

The most common F-measure is the F_1 measure, where the beta parameter
is set to one. However, by varying beta we can give more weight to
either precision or recall, depending on which one we subjectively feel
is more important.

There is not much agreement about what value is better or worse for
beta, so most people leave it at one, which treats recall and precision
as equally important. Two other somewhat common values for beta is 0.5
(more emphasis on precision than recall), and 2 (more emphasis on recall
than precision). Fore more information see the Wikipedia entry [F1
score](https://en.wikipedia.org/wiki/F1_score).