gm background option matcher is too strict

[Munter]

https://github.com/papandreou/express-processimage/blob/master/lib/getFilterInfosAndTargetContentTypeFromQueryString.js#L90

background uses the fill syntax:

    name               (named color)
    #RGB               (hex numbers, 4 bits each)
    #RRGGBB            (8 bits each)
    #RRRGGGBBB         (12 bits each)
    #RRRRGGGGBBBB      (16 bits each)
    #RGBA              (4 bits each)
    #RRGGBBAA          (8 bits each)
    #RRRGGGBBBAAA      (12 bits each)
    #RRRRGGGGBBBBAAAA  (16 bits each)
    rgb(r,g,b)         (r,g,b are decimal numbers)
    rgba(r,g,b,a)      (r,g,b,a are decimal numbers)

See http://www.graphicsmagick.org/GraphicsMagick.html#details-fill

[Munter]

Color names are the one provided in gm convert -list color

[papandreou]

I guess /^[A-Za-z]+\d*$/ will be sufficient?

[Munter]

I'm not entirely sure of gm's behavior with invalid inputs. Throwing would not be nice. Silent error probably also not so nice. Only matching on a whitelist would at least give a possibility to discover that the option wasn't recognized (how I noticed the missing syntaxes in the first place)

[papandreou]

I'm not entirely sure of gm's behavior with invalid inputs

Try adding a test :)