Adds `less-css-input-file-name` option; Changes `compile` to `shell-command`

The purpose of `less-css-input-file-name` option is to make possible to compile another file on save. It was useful for me, when I edited `variables.less` that was included in `style.less`, so I actually needed `style.less` to be compiled to `style.css`.

Also I found it quite annoying that `*compilation*` buffer opened every time on save :) I didn't find a way to prevent this behaviour of `compile` function, but I learned that `shell-command` wouldn't open an extra buffer if command exited with no output. Though it should open a buffer if there were errors or warnings.