Add ChannelEditComplex

[psheets]

Add ChannelEditComplex function and ChannelEdit struct.

Allows updating a channel's fields such as topic.

Implemented as suggested by @iopred in #322 and #492

[Seklfreak]

I think all possible attributes should be added. ( https://discordapp.com/developers/docs/resources/channel#modify-channel )

Missing right now: position, user_limit, permission_overwrites and parent_id.

[psheets]

ChannelEdit struct has been updated to include all fields per @Seklfreak comment.

[iopred]

2 things:

1. The file encodings changed again
2. Modify the existing ChannelEdit method to call the new ChannelEditComplex method

Thank you so much for this PR!

[Strum355]

Re. point 1, I havent heard them being called "file encodings", so for those who havent heard this, its the files permissions (as per *nix) that have been changed from 644 to 755. On that note, the image most definitely shouldnt need to be 755

[iopred]

Ah, sorry I didn't look closely, I've also seen this when the newline character (\n -> \r\n) changes, which is why I said encoding.

[Strum355]

I ran the files and its current live one through dos2unix and cmp, no difference in encoding, so it must be just a permissions thing ^^ Still no need for the change in permissions, so that should be reverted, yea

[psheets]

Reverted permissions back to 644 and Modified ChannelEdit to use ChannelEditComplex per @iopred and @Strum355

[iopred]

Sorry to be a pain, could we change this method signature to:

ChannelEditComplex(channel ID string, c *ChannelEdit)

We can then remove the ID field from the struct.

This would bring this API closer to the ChannelMessageSendComplex method.

Thanks!

[psheets]

Not a pain, thank you for the feedback. This is a learning experience for me.

The current signature is mimicking ChannelMessageEditComplex:
func (s *Session) ChannelMessageEditComplex(m *MessageEdit) (st *Message, err error)

and the MessageEdit struct:

type MessageEdit struct {
	Content *string       `json:"content,omitempty"`
	Embed   *MessageEmbed `json:"embed,omitempty"`

	ID      string
	Channel string
}

Should ChannelMessageEditComplex be rewritten to be inline with your suggestion?
Adding channelID seems to match the other functions.

[psheets]

I see that changing ChannelMessageEditComplex will break the existing API.

[iopred]

Thanks so much!