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
Done; this was too hard. We don't use commanderjs the way it was intended. It wants to set properties on the command line object while we want those properties set on a separate Options object. (The commanderjs approach is dubious to be sure).  This mostly works except when processing a non-boolean option that can have boolean aspects.  For example `--source-maps` can have values [file | inline | null] or be absent. The default value is false but a null means the option should true. Threading this logic through commanderjs plus or code is trouble.