implement plugin system

[aaronjanse]

Sibling PR: https://github.com/sifive/wit-scala-plugin/pull/1

Future todo:

• add tests to sifive/wit-scala-plugin
• document plugin system

Area of interest:

wit/lib/wit/main.py

Lines 194 to 198 in 42904d3

	elif args.command == 'fetch-scala':
	log.error("To install the scala plugin, run:\n"
	" wit add-pkg https://github.com/sifive/wit-scala-plugin::"
	"9246f3400b8fab6eccc828981026c9947d4a1b0c\n"
	" wit update")