Implemented the four widget related endpoints of the api. Partial user
object being returned as part of the widget.json endpoint [1] is an
intentional choice related to privacy [2].

The widget.json endpoint will require additional changes upon completion
of other work. Member details will need to return extra key/values for
connected users to voice channels. An additional avatar_url value will
hold an unique avatar url for the user + guild, fetched via a CDN
endpoint widget-avatars.

New dependencies `canvas` and `image-size`. Canvas is used to create the
widget.png endpoint image [3]. Image-size is used to set the canvas'
size to match the widget template images.

Use regex in determining if a NO_AUTHORIZATION_ROUTES is hit or not.

[1] https://discord.com/developers/docs/resources/guild#get-guild-widget
[2] https://github.com/discord/discord-api-docs/issues/1287
[3] https://discord.com/developers/docs/resources/guild#get-guild-widget-image

Closes: #9, #110