Allow specifying substitution characters

[sean-parent]

Currently the docs for a file like <lib/file.hpp> turns into a URL like lib2Ffile.hpp - it would be great to either allow lib-file.hpp or lib/file.hpp - similarly other special characters should be substituable to something more readable.

[fosterbrereton]

Finding the right substitution characters can be tricky, and many options result in documentation collisions. For example, I tried - at one point, but then documentation for, say, operator- and operator* will end up resolving to the same thing. I only arrived at the current solution after many collision failures. I agree what's there now is not ideal, and am open to better solutions.