Resolve source maps for input files in multiple ways

[connorjclark]

Related to #3058

Currently, there is no way to provide inputs such as this:

src/
- file1.js
- file1.js.map
- file2.js
- file2.js.map
- file3.js

file1.js

console.log('hello');
//# sourceMappingURL=file1.js.map

file2.js

console.log('goodbye');

file3.js

console.log('idkyu');
sourceMappingURL=data:application/json;charset=utf-8;base64,...

And have uglifyjs compose all the input files' source maps together.

The source-map.content option allows for inline, which will correctly compose files like file3.js. The option also also allows for giving the actual source map JSON via the CLI, which I believe only works for a single input file.

I am proposing (and am currently working on) an option for source-map.content called auto, which will attempt to resolve source maps in this way:

1. If sourceMappingURL is specified, decode the base64 if the source map is inline. If the source mapping url is a resource location (either a file on disk or an HTTP asset), fetch it.
2. If there is no sourceMappingURL, and the input file name is file.js, look for the source map on disk at file.js.map and file.map