feat: add gnuplot support

[sahashirshendu]

The default commentstring for gnuplot files (.plt, .gnu etc.) does not work. But hash type does.

[numToStr]

Thanks for the PR. Just one nit, cmake also has same block commenstring perhaps we could make it common in the M table as cmake_b or hash_bracket.

[sahashirshendu]

cmake also has same block commenstring

The block commentstring does not work quite completely in gnuplot, it works for a single line but not for multiple lines (an actual block); but is still is better than the default one which works nowhere :)

perhaps we could make it common in the M table as cmake_b or hash_bracket.

Just did that.