Use equal sign instead of space between option flag and value

With respect to https://github.com/gruntjs/grunt-contrib-sass/issues/140, https://github.com/gruntjs/grunt-contrib-sass/issues/142 and https://github.com/sass/sass/issues/1321,
use `=` instead of space flag and value delimiter, i.e. output `['--foo', 'bar']` becomes `['--foo=bar']`.