fix: calling parse multiple times now appropriately maintains state (#1137)

[mleguen]

Extends the freezing/unfreezing mechanism already in place for parse(args, cb) to parse() (without arguments) to be able to call it several times with the same behavior.

Also corrects the builder function return value handler for it no longer to depend on yargs.parsed being set after a call to parse() (which is no longer the case after unfreezing).

Closes #1137

[bcoe]

@mleguen I'll definitely have to look at this work closely, years since I touched these bits 😄

but this sounds like awesome progress.