feat: add coerce api

[nexdrew]

Closes #535.

Builds on top of yargs/yargs-parser#42 and yargs/yargs-parser#47.

Adds a .coerce() method that allows users to map a coercion (or transform) function to specific option keys. Coercion is then delegated to yargs-parser.

The new method accepts the following:

• a single key and a function
• an array of keys and a function
• an object mapping keys to functions

The .option() method was also augmented to support a coerce property.

Note that, in commit 46eaa9e, a special case of coercion logic was needed to support positional and variadic args for commands. This is because yargs-parser does not populate positional args in argv - those are populated in lib/command.js after parsing is complete - and so it was necessary to duplicate the execution of coercion functions after population of positional args. I would eventually like to replace this by moving the positional arg population logic down to yargs-parser, but that's a bigger change.

Also note that, when mixing default values with coercion for positional args, coercion is actually executed twice - first in yargs-parser against the default value and second in yargs against the "parsed" value. Similar to yargs/yargs-parser#51, variadic args are given to the coercion function as an array (function called once) instead of as individual element values.

• Add .coerce() to API
• Support for coercion of positional and variadic args for commands
• Tests
• Add docs to README