Fix Filter parsing

[moyogo]

As mentioned in googlefonts/ufo2ft#238:
We need to change the way filter strings are parsed in glyphLib. Width:+30, which should add 30 to the width, doesn’t mean the same asWidth:30 or Width:=30 or Width:=+30, which should set the width to 30.

Width:-30 should substract 30 from the width, while Width:=-30 should set to the width to 0 since width cannot be negative (RSB:=-30 and LSB:=-30 do set RSB and LSB to -30).

[anthrotype]

the function glyphsLib.builder.filters.parse_glyphs_filter doesn't know how to deal with the equal sign.
Currently when it sees a string that looks like a number, it casts it to one, using this other function cast_to_number_or_bool.
In the case of a value that is prefixed with the equal sign, it keeps the value as a string.
Maybe that's ok. The ufo2ft Transformations filter can check if the value is a string and starts with an equal sign, then treat it accordingly.
WDYT?

[belluzj]

(not an answer to your question, sorry! In cast_to_number_or_bool:
https://github.com/googlei18n/glyphsLib/blob/60026c83af4d02b23470d770a66868092d97c1f3/Lib/glyphsLib/util.py#L66
This is another place where the glyph name "infinity" might become the float +Infinity)

[anthrotype]

good catch

[davelab6]

This is needed by Google Fonts for being able to generate "sibling families" using fontmake (described in #355 (comment))