support for config files

[vito]

currently flags like --html-templates and --plugin have to be passed on every command. maybe these could be in a config file instead? then we wouldn't need to keep writing scripts like scripts/build-docs.

counter-argument: writing a script already works and is almost the same amount of work

counter-counter argument: it's a little more awkward to write docs saying 'write a script' vs 'put this in your config'