Add option to output JS Maps, add generic support for iterables

[eemeli]

Fixes #46, fixes #49, fixes #50

On the YAML -> JS side, this adds a new option mapAsMap, with which YAML mappings are represented in JS as Map rather than Object instances:

YAML.parse('{[1, 2]: many}')
// { '[1,2]': 'many' }

YAML.parse('{[1, 2]: many}', { mapAsMap: true })
// Map { [ 1, 2 ] => 'many' }

The specific benefit of this is the ability to natively use non-string keys in mappings.

Then on the JS -> YAML side, iterable values are now supported by createNode(). The default is to parse them as sequences, but Map and its descendants are parsed as mappings.

[eemeli]

I'm considering adding a detector for Immutable.js maps as well, with something like iterable.constructor.isMap && iterable.constructor.isMap(iterable) as an alternative for iterable instanceof Map. Any thoughts on whether that would make sense, or would it be too specific? Checking iterable[Symbol.iterator].name === 'entries' might work as well, but it's a bit flaky.