Add property marker-symbolcolor for marker text and symbol color

[Lapizistik]

Rationale: the spec does not define in which color the marker-symbol
should be drawn. The spec clearly states that stroke and fill styles
shall not affect marker styles, so stroke cannot be used for this (and
the defaults of stroke and marker-color would not work well for this).

[jfirebaugh]

There's already a property to specify the color of the marker: marker-color. Why do we need an additional marker-symbolcolor property?

[Lapizistik]

The marker color (as far as I understand the term) describes the main color of the marker (i.e. the fillcolor of the marker shape, not the color of the symbol or text printed on this background color.
Some applications default the symbol color to white, which does not work well with a white or light background. Others let you choose both colors:

• https://raw.githubusercontent.com/marslan390/BeautifyMarker/master/images/img-demo.PNG: You have yellow markers with red symbols, white Markers with blue symbols and so on
• https://camo.githubusercontent.com/139673d887bedbf26e286ebfc5fb40d139f879f9/68747470733a2f2f7261772e6769746875622e636f6d2f636f72796173696c76612f4c6561666c65742e45787472614d61726b6572732f6d61737465722f73637265656e73686f742e706e67:

Contrary (only white symbols):

• • https://camo.githubusercontent.com/a0281bc5c0b55f8175fd3ed18c972b320c60e254/68747470733a2f2f7261772e6769746875622e636f6d2f6c766f6f6764742f4c6561666c65742e617765736f6d652d6d61726b6572732f6d61737465722f73637265656e73686f74732f73637265656e73686f742d736f66742e706e67

I am currently working on a library which (as one feature) can show geojson with simplestyle-spec support on a map, and I support different marker-background and marker-symbol colors. I just thought it would be nice to do this following some spec ;-)