Implement inlineSourceMaps option.

Add sourceMapURL method to Compiler, call btoa when option is set.
Override in NodeCompiler with node version.
Add test for node option --inline-source-maps.
Verify browser via repl.

Based on suggestions by @jeanlauliac and @guybedford
Fixes #978 Fixes #892 Fixes #1393
Just read it a little more clearly and see you already do this elsewhere - is there a reason you can't use Buffer in the browser?