Add auto-complete to channels, mentions, and emoji.

[iProgramMC]

Closes #100.

Tasks:

• [x] Add auto-completion base. It can substitute channel names.
• [x] Split out the autocomplete code into its own class. This allows for flexibility later.
• [x] Add support for mentions in the active server
• [x] Add support for emojis in the active server
• [x] Highlight the selection in a better way
• [x] Make DiscordInstance's SendMessage functions resolve channel, user name and emoji references

Note that this list of tasks is incomplete and items may be added.