Allow importing configuration

[brandonchinn178]

Support an import key that would allow merging config from another file. Useful for monorepos, e.g.

# tasty-autocollect.conf
group_type = flat
strip_suffix = Test

-- package1/test/Main.hs
{- AUTOCOLLECT.MAIN
import = ../../tasty-autocollect.conf
suite_name = package1
-}

-- package2/test/Main.hs
{- AUTOCOLLECT.MAIN
import = ../../tasty-autocollect.conf
suite_name = package2
-}