added nodeMirror to editors list

let me know if this is appropriate
Merge pull request #157 from soliton4/patch-1

added nodeMirror to editors list
editors-and-plugins.md: polished nodeMirror description.
Update frameworks-using-less.md

Added my mixin library to the list
Merge pull request #158 from pixelass/patch-1

Update frameworks-using-less.md
Updated using-less.md with some links.
Documented `+_` property merge operator.
Merge pull request #159 from seven-phases-max/space-property-merge

Space property merge
Merge pull request #156 from seven-phases-max/master

Updated using-less.md with some links.
Add info for PhantomJS
Update color-channel.md
Merge pull request #162 from bassjobsen/patch-6

Update color-channel.md
color-channel.md: some minor polishing of luma/luminance.