Enhancement: Enable expedient solc-js parsing when using docker or native solc

[CruzMolina]

Continuation of #2007 building on the efforts of #2042.

Context:
Removing solc-js from Parser.parseImports significantly slowed docker compilations.

Solution:
This PR enables leveraging the default cached soljson (currently solc 0.5.8) specifically for speedy import parsing when using docker or native solc. Primary contract compilation is still handled by whatever the user is specifying.

To enable speedy JS parsing, users will need to specify a parser: "solcjs" key/value pair in their config like so:

module.exports = {
  compilers: {
    solc: {
      parser: "solcjs"
    }
  }  
}

[coveralls]

Coverage decreased (-0.02%) to 70.372% when pulling 512a5fb on enableJSparsing into 238c5c0 on develop.

[CruzMolina]

Wow, thanks for that long detour Travis.

@gnidan, I think this is finally ready.

[BrendanChou]

Any plans to merge this?

[gnidan]

Ah thanks for the ping @BrendanChou. I think this was left in a waiting state while #2109 was in flight, but this might be good now? @CruzMolina this can be reviewed/merged pending the conflicts, right?

[gnidan]

Let's get this released!