Add parser support for 3-letter month

[WithoutPants]

Adds an {mmm} parser field to the scene filename parser. Matches against three letter month such as Jan (case-insensitive).

[bnkai]

Looks ok, works ok also.
A small observation
If month is supported like this shouldn't the day also be supported in single digit?
Apr.5.2020 for example is more common in this notation than Apr.05.2020

[WithoutPants]

We currently have {d} as delimiter. This can easily be changed to another character.

We really want either {dd} to accept one or two date characters, or have the new {d} field accept one or two date characters. A single-only date field would be of pretty limited use. I am leaning towards using a new {d} single/double date field currently. That said, given that the design on this isn't straightforward, I think we can push this to a separate PR rather than creeping the scope of this one.