Add nowarn=:allexcept and except=[f, g, h, ...] options to @check and check

[DilumAluthge]

Pull request #32 added the ability to tell @check/check to throw an error if any warnings occur inside any function.

This pull request extends this functionality by adding the ability to tell @check/check to throw an error if any warnings occur inside any function EXCEPT those functions listed in the except array.

Before this pull request

1. @check f(x) nowarn=[g]: throw an error if any warnings occur inside the function g
2. @check f(x) nowarn=:all: throw an error if any warnings occur inside any functions

After this pull request

1. @check f(x) nowarn=[g]: throw an error if any warnings occur inside the function g
2. @check f(x) nowarn=:all: throw an error if any warnings occur inside any functions
3. @check f(x) nowarn=:allexcept except=[h]: throw an error if any warnings occur inside any functions EXCEPT the function h

[DilumAluthge]

Travis failures on Julia 1.2 and nightly are known (issue #31)

[pfitzseb]

The nowarn arg is redundant when except is supplied, or am I missing something?

[DilumAluthge]

The nowarn arg is redundant when except is supplied, or am I missing something?

Yeah that’s a good point. So if the user supplies except, should we just ignore the value of nowarn, whatever that value is?

[DilumAluthge]

Alright, I've updated the code. If except is nonempty, I ignore the value of nowarn.