feat: add dynamicTypes option to use function when looking up types

[lukekarrys]

Note: this is stacked on #124 since it had some potential conflicts. That one should land first.

This PR adds an option named dynamicTypes to go along with the existing types option.

Currently, nopt looks up each potential key in the types object. If types doesn't have a property by that name, then the current fallback is to parse it as a boolean and leave the value in argv.remain.

With dynamicTypes, you can specify a function that will receive each unknown key and can either return a type or undefined to keep the current behavior.

Here's an example:

const args = [
  '--known', '100',
  '--unknown', '50',
  '--other', '1',
]
nopt.nopt(args, {
  types: { known: Number },
  dynamicTypes: (k) => {
    if (k === 'other') {
      return Number
    }
  },
  typeDefs: nopt.typeDefs,
  shorthands: {},
})
/* {
  known: 100,
  unknown: true,
  other: 1,
  argv: {
    remain: ['50'],
    cooked: ['--known', '100', '--unknown', '50', '--other', '1'],
    original: ['--known', '100', '--unknown', '50', '--other', '1'],
  },
} */