fix: only run coercion functions once, despite aliases. (#76)

[Mike111177]

Fixes #76
Fixes yargs/yargs#923

[bcoe]

@Mike111177 wouldn't this mean that if foo and bar are aliases, only one of either foo or bar has the appropriate final value? or am I missing something.

[Mike111177]

@bcoe, no, note lines 523 and 524. Once a coerce function runs, it applies the value to all needed aliases. Then on the next rounds it ignores the aliases that have already been assigned to.