Add TransversingCSS module to allow tests with CSS selectors

[amitaibu]

fixes #1735

1. Currently, it's a shameless copy/ paste from here and here
2. It Would have been nicer if Yesod had exposed it as an importable package. Makes sense? If so, I'll open an issue there.
3. html-conduit seems to be needed. Did I add it in the correct places? How to re-build IHP with this new package?

[mpscholten]

It Would have been nicer if Yesod had exposed it as an importable package. Makes sense? If so, I'll open an issue there.

Yes, that seems useful to me 👍

html-conduit seems to be needed. Did I add it in the correct places? How to re-build IHP with this new package?

In ihp.nix it needs to be used somewhere else a bit lower in the file as well, so it should be two places where it's added in ihp.nix (once to declare it as the dependency at the top level of the file, and then somewhere in the file to actually use it)

[amitaibu]

Created issue on Yesod - yesodweb/yesod#1810