Feature request: color for the <ins> and <del> tags

[ericniebler]

i would love to use the <ins> and <del> tags to denote inserted and deleted text. GFM allows them and uses the default CSS styling for them: underline for <ins> and strikethrough for <del>.

i find this styling very subtle and easy to overlook. in standardization work, it is customary to also change the color of inserted and deleted text: inserted text is green and underlined, and deleted text is red and strikethrough. the following is taken from an ISO C++ Committee paper for example:

if GFM styled inserted and deleted text in this way i could use <ins> and <del> without worrying that the meaning will be overlooked.