Refactor flowconfig option parsing into a list of parsers
Refactor flowconfig option parsing into a list of parsers

Summary: 1) more succinct, 2) enables a declarative list of options, which can be used for generating docs or `flow config get <KEY>`

Reviewed By: dsainati1

Differential Revision: D12983297

fbshipit-source-id: 57977f0282f5a3a586dc6b455952ce0575f2bfe5

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow