Support parsing of synopses

[lenormf]

The reference[1] defines synopses as follows:

Synopses are purely optional tools for the writer's outlining
and organizational process. They are ignored in formatted
output.

This commit makes the Foutain parser aware of them, even though the
export classes ultimately ignore them.

[1] https://fountain.io/syntax#section-overview

[vilcans]

What does this patch do? The parser already supports synopses (although it's called synopsis, as that is the singular form of synopses; I just noticed that the incorrect plural form "synopsises" is used at one function name though).

[lenormf]

I didn't notice the existing code because there was no regex for synopses.

Without my patch, synopses are inserted into the output.