Remove redundant arguments check

`arguments` is always truthy inside a function, so these checks don't do
anything, but any function that references raw `arguments` will be
de-optimised (at least in V8), so it would be better to remove the
check.
Define a variable for `progressPlugin`

`progressPlugin` was defined in the global scope by accident
Merge pull request #63 from ForbesLindesay/redundant-args-check

Remove redundant arguments check
Merge pull request #64 from ForbesLindesay/fix-undefined-progressPlugin-variable

Define a variable for `progressPlugin`