Add command line options for binding category selection to literate mode

[edsko]

Since literate mode, like TH, is constrained to producing just a single file, it could be rather useful to have command line options to choose between safe, unsafe, or pointers, or indeed a combination thereof, though if a combination is desired, renaming must be done; probably a command line option for adding a prefix or a suffix would suffice.

[edsko]

Note that, like in TH mode, users could generate multiple modules by invoking bindgen multiple times, but then it's even more important that they can at least choose safe/unsafe/pointers; if they want to combine them into a single file, then renaming is also needed.