Add option to inhibit upcase for wiki links in gfm-mode

[shipmints]

Greetings and thank you for the markdown package (and also deft, which I also use).

I find the auto upcase of the initial letter of a wiki link file to be kind of annoying, and imprecise when the linked-to file already exists and does not start with an upper-case letter. Strict compatibility in this regard with Github does not seem mandatory (and, of course, only matters if one also uses Github for the same content).

I'd like to propose an option, perhaps called markdown-wiki-link-inhibit-upcase, to suppress this behavior. I'd happily advise markdown-convert-wiki-link-to-filename to override upcase, but upcase is not advisable when byte-compiled.

I'd be happy to provide a PR.

[shipmints]

I decided to implement the flag as markdown-wiki-link-retain-case to avoid changing case at all, respecting the user's input. PR incoming.

[shipmints]

Thank you.