Allow using any reader for PageType

[singpolyma]

Instead of a closed list, allow anything Pandoc understands. If there is no
markup/markupHelp file for the reader, then there will be no help, otherwise
this works as before.

Instead of special-casing +lhs this is now just a normal extension specification
understood by Pandoc. +emoji is always added as before. Reader specs are run
through toLower for compatibility with the existing system.

Closes #636
Closes #580

[tim-janik]

Thanks for this PR, I also need to set Pandoc flags in addition to "markdown".

Locally, I've rebased the changes onto 0.15.1.0 and tried to build it.
I have Ubuntu 22.04 with pandoc 2.9.2.1 here, but still I needed to disable the condition
#if !MIN_VERSION_pandoc(2,12,0) in order for Data.Text to be qualified as T in src/Network/Gitit/Util.hs.
Also, I had to restore the data files with upper case extensions for the build to succeed.