Add argument to turn off suppression of warnings and messages

[jimhester]

Closes #244

[jimhester]

Thinking about this a little more this isn't exactly the API I would prefer. While it does allow you turn turn off suppression for each test_that block it does not allow you to turn it off globally.

I may change the default value suppress = getOption("testthat.suppress", c("message", "warning")) which would let you change them globally.

Edit: I did do what I suggested with 0b59ab1

[hadley]

I think a global option is the wrong way to do this.