prepare could take a json file as argument

[hswick]

Right now use of prepare.js can involve a lengthy amount of arguments since we use absolute paths and there are usually around 5 arguments to give.

Thoughts are we could add a --json flag that accepts a path to a json file serving as a configuration for the task.

We could replace:

node /truebit-toolchain/modules/emscripten-module-wrapper/prepare.js \
/workspace/reverse_alphabet/reverse_alphabet.js \
--file /workspace/reverse_alphabet/alphabet.txt \
--file /workspace/reverse_alphabet/reverse_alphabet.txt \
--asmjs \
--out /workspace/dist

With:

node /truebit-toolchain/modules/emscripten-module-wrapper/prepare.js --json /workspace/reverse_alphabet/task_config.json

task_config.json looks like:

{
     source: "/workspace/reverse_alphabet/reverse_alphabet.js",
     files: [
         "/workspace/reverse_alphabet/alphabet.txt",
         "/workspace/reverse_alphabet/reverse_alphabet.txt"
     ],
     asmjs: true,
     out: "/workspace/dist"
}