Handle parentheses in variables in commands

[i7an]

Re-created #526.

Variables with parentheses are not handled correctly in commands:

FOO='passwo(rd'
BAR=$(echo '$FOO')

Result:

expected: {"BAR"=>"passwo(rd",           "FOO"=>"passwo(rd"}
     got: {"BAR"=>"$(echo 'passwo(rd')", "FOO"=>"passwo(rd"}

The solution I suggest is to parse commands first and expand variables before executing commands.

[bkeepers]

Thanks @i7an. Sorry for the delay on your previous PR.

I'll get this released shortly.