fix: Use decamelize from npm instead of vendored copy

[coreyfarrell]

@bcoe yargs-parser already depends on decamelize@1.2.0 (and it's always require()'ed). So this doesn't add anything to the node_modules directory and it avoids loading two copies of decamelize into memory by making yargs and yargs-parser use the same module.

Note that decamelize@3 is available (requires node.js 6). This is not used because it pulls in xregexp for unicode regex support (9.5MB!!). xregexp will be dropped in a future version when node.js 10 is required.

[bcoe]

@coreyfarrell this is also landed in yargs@next.