Add options to control indentation

[eemeli]

Closes #143
Related: #53

This adds the options indent: number and indentSeq: boolean, to allow for indentation to be customised. The defaults are { indent: 2, indentSeq: true }.

indent is the number of spaces by which each indentation level is prefixed, and should be a positive integer.

If indentSeq is set to false, the '- ' prefix of block sequence items is considered to be a part of the indentation where possible. This requires indent >= 2, and won't apply to sequences with anchors or explicit tags.

const seq = YAML.createNode(['a'])
seq.commentBefore = 'sc'
const map = YAML.createNode({ foo: 'bar', seq })
map.commentBefore = 'mc'
const obj = { array: [{ a: 1, b: 2 }], map }

YAML.stringify(obj, { indent: 1 })
// array:
//  - a: 1
//    b: 2
// map:
//  #mc
//  foo: bar
//  seq:
//   #sc
//   - a

YAML.stringify(obj, { indent: 4, indentSeq: false })
// array:
//   - a: 1
//     b: 2
// map:
//     #mc
//     foo: bar
//     seq:
//         #sc
//       - a

[eemeli]

Rebased to include updates to documentation.