Color aliases for custom color support

[blacksails]

#1095 introduced custom color support. This PR improves the implementation, so that it doesn't rely on entering entire escape codes but instead it uses the color names and for the 256 color palette it uses their number.

Example of how this feature improves usablity when using a color scheme like solarized. Before this change the colors looks like this when using the solarized color scheme

By setting the following envvars

export GINKGO_CLI_COLOR_RED="red"
export GINKGO_CLI_COLOR_ORANGE="bright-red"
export GINKGO_CLI_COLOR_CORAL="bright-magenta"
export GINKGO_CLI_COLOR_MAGENTA="magenta"
export GINKGO_CLI_COLOR_GREEN="green"
export GINKGO_CLI_COLOR_DARK_GREEN="green"
export GINKGO_CLI_COLOR_YELLOW="yellow"
export GINKGO_CLI_COLOR_LIGHT_YELLOW="yellow"
export GINKGO_CLI_COLOR_CYAN="cyan"
export GINKGO_CLI_COLOR_GRAY="bright-blue"
export GINKGO_CLI_COLOR_LIGHT_GRAY="bright-cyan" 
export GINKGO_CLI_COLOR_BLUE="blue"

The output now looks like this

One could also chose to use the color numbers like (which also is needed to reference the 256 color palette).

export GINKGO_CLI_COLOR_RED="1"
export GINKGO_CLI_COLOR_ORANGE="9"
export GINKGO_CLI_COLOR_CORAL="13"
export GINKGO_CLI_COLOR_MAGENTA="5"
export GINKGO_CLI_COLOR_GREEN="2"
export GINKGO_CLI_COLOR_DARK_GREEN="2"
export GINKGO_CLI_COLOR_YELLOW="3"
export GINKGO_CLI_COLOR_LIGHT_YELLOW="3"
export GINKGO_CLI_COLOR_CYAN="6"
export GINKGO_CLI_COLOR_GRAY="12"
export GINKGO_CLI_COLOR_LIGHT_GRAY="14" 
export GINKGO_CLI_COLOR_BLUE="4"

This moves the custom color support away from the raw ASCI escapes and instead uses words for the standard 16 terminal colors or optionally 0-255 for the extended 256 color palette.

Besides the feature improvement a missing envvar unset in the tests was added.

fixes #1098

[onsi]

looks good to me!