Add compiler flag to suppress implicit conversion warnings

[phoney]

After recently importing the current version of cmark one compiler warning was fixed but five new ones appeared. I think this kind of whack-a-mole problem will continue as long as we're using cmark since it's C language and Down has different warnings enabled than the cmark developers do.

I've added a compiler flag to ignore the implicit integer conversion warnings. These are benign warnings and there's nothing really to be done about them in Down. When this warning is disabled Down compiles without warnings.

[coveralls]

Coverage remained the same at 90.625% when pulling 23076b4 on phoney:feature/fix-warnings into 1c90cbb on iwasrobbed:master.

[iwasrobbed]

Looks good, thanks!

[iwasrobbed]

We'll see if anyone has issues with this and if we need to move it into a pod_target_xcconfig type specifier to scope it to just the pod and not the whole project. I know there's an inhibit_warnings! flag you can set in your Podfile as well but not sure if it works on this or not