Implement support for RFC 8, transitive NPM dependencies

[alexcrichton]

Specified upstream RFC 8 is a way for NPM dependencies to work transitively throughout the crate graph. The first phase of supporting this is now implemented in wasm-bindgen and the next phase is in wasm-pack itself. Thankfully it shouldn't be too hard!

Currently today wasm-pack will write out a package.json, but this will overwrite a package.json created by wasm-bindgen, if one exists. Instead what wasm-pack should do is:

• When creating package.json, first check to see if one already exists at the target location
• Deserialize the dependencies map, if it exists, otherwise start with an empty object
• Add in all the current configuration (name, scope, urls, etc)
• Write out the package.json again, overwriting what was previously there.

Basically the package.json that wasm-pack writes should just be merged with the package.json emitted by wasm-bindgen if it's already there.

[Tarnadas]

Hey @alexcrichton,

Let me know what you think about my PR #732

[trivigy]

The restriction in the RFC which restricts package.json to only one key dependencies builds the entire build process because of THIS. Would greatly appreciate some advice on how to work around this restriction.