New rcParams to set pyplot.suptitle() defaults

Two new rcParams are introduced:

- figure.titlesize (xx-large)
- figure.titleweight (normal)

Allow to set the pyplot.suptitle() default font size and weight.
Add a "whats_new" entry for the new figure.title* rcParams.
Merge pull request #3083 from wavexx/suptitle

ENH : New rcParams to set pyplot.suptitle() defaults