Throw an exception when creating a Color with an invalid float value

Conflicts:
	src/Discord.Net.Commands/CommandService.cs

For a command to use the DefaultRunMode, it must now have it's RunMode set to RunMode.Default (this is the default value on CommandAttribute now).

This prevents a 400 when sending a malformed color to Discord.

ModifyAsync will accept an EmbedBuilder

Add CommandServiceConfig, DefaultRunMode

**This is not a breaking change**.

This change begins to implement #379, where IUserMessage.ModifyAsync will now accept a `Discord.ModifyMessageParams` func over a `Discord.API.Rest.ModifyMessageParams` func.

This adds an (optional) CommandServiceConfig, as well as a DefaultRunMode for commands.

This resolves #368 (for commands where a RunMode is not explicitly specified, a custom default value should be used)

This is a breaking change.

Adds a precondition that requires the bot has a specified permission (Resolves #211).

Renames RequirePermission to RequireUserPermission.

This precondition will require that the invoker of the command is the owner of the bot.

Builders for Thumbnail and Image embeds

This should Fix #262

Updating CDN url

This probably isn't all of them. This is what I get for copy+pasting code.

In theory this should just work, more testing is needed though

Values in ImmutableDictionaries were strings rather than enumType