Add custom string formatter for lower/upper support

[djhoese]

See pytroll/satpy#417

I'd like to add a custom formatter so that someone could call upper and lower and possibly other operations on a string to help "standardize" filenames the way users want without having to directly modify the attributes/parameters going in to the string formatting.

[djhoese]

Skimming the Parser code which is used in satpy to accomplish this: we'll have to note that these functions are only used for formatting the string and do not affect the glob/pattern matching functionality. @pnuu, sound good?

[pnuu]

Yep, sounds right.