add `--pure-funcs` option

add `--pure-funcs` option

it has the same effect as specifying `pure_funcs` in `--compressor`
option, however it's much easier to use

instead of:

    --compressor 'pure_func=["Math.floor","debug","console.logTime"]'

it's now possible:

    --compressor --pure-funcs Math.floor debug console.logTime

fixes #684