Support `--flag=value` syntax

[phoenixr-codes]

The --flag=value syntax is used by many CLIs and has the benefit of beeing more concise in some contexts. For example: prog -f=bar foo vs prog -f bar foo. In the first example it's clear that bar is part of the flag f. Note that flags that accept multiple values may be problematic. A solution would be to adopt the syntax -f=value1,value2,value3.