Create `valueFormatter` for monetary RA, images about psychtaskframework

[shippy]

Two possible cases.

1. Transform a value into a particular format, e.g. -5 -> -$5.00
2. Look up a value in a table to produce a text and/or image.

So long as pie-in-the-sky #26 remains unimplemented, the task-drawing functions aren't general, and so they'll know:

1. what formatters to use for the job,
2. which lookup table from config they should supply.

Still, the suggested location for lookup tables in config is s.objects.X.labels.txt = {} and s.objects.X.labels.img = {}. For texture pre-loading purposes (see #5 and loadTexturesFromConfig), the latter field name is compulsory.

(This will require rewriting extractImagesFromConfig, as the function currently expects a character array in .img fields, not a cell array of character arrays.)

[shippy]

Better suggested location: s.lookups.X.txt and s.lookups.X.txt.