Add option to use relative path for compiler arguments

[nandenjin]

This is a feature request.

Motivation

It's nice if we can use more various compiler command like Docker container emscripten/emsdk.

Problem

Currently, all paths passed to compilers are absolute, so compilers that take files in virtual file systems are not able to be used.

Proposal

Give relative paths for compilers and set cwd for its process (or add options to do so).

• It is useful to compile various commands, not only emcc itself. Ex: docker run emscripten/emsdk emcc
  • (Actually, to use compiler on Docker we have to add Accept commands in addition to paths for compiler for options #9 . I've just submitted a PR feat: Accept commands and paths for options #12 for it.)