Fixes #213 by allowing the library user to explicitly specify the token type if they wish to do so.

I.E., authToken previously worked as follows:

• Prefix input with "Bot "

Now, authToken works as follows:

• If there is already a prefix in the input, e.g. "Bearer " or "Bot ", use it
• Otherwise, prefix input with "Bot "

I've documented the two types of tokens briefly, but a more in-depth explanation may need to be added somewhere more visible (e.g. in the README or somewhere linked from it), see #213 (comment) for more info.