Add custom domain support (#103) (#108)

Squashed commits:

- Fix example code in readme
- Add example for a custom API domain
- Remove unused config variable "apiRegionAsSubdomain"
- Removed proxy documentation
- Rename ApiURL variable to fit code style
- Add support for region as a header parameter, and more specific URL building
- Moved RegionConfig to Camille.RiotGames.Enums, and changed its values casing
- Remove generated license callout
- Correct variable in apiUrl description
- Clarify apiUrl description
- Move RegionHeaderKey to instead be RegionKey, and use it for the query parameter
- Use the new RegionKey variable
- Do not branch off of the URL contents
- Use RegionKey to build the URL without string checking
- Change naming region->route
- Simplify query param editing code (.NET is a mess)