Add support Custom profile fields

[Tozapid]

Description

It will be great if we add support for custom profile fields

(Optional) Slack's documentation

API: https://api.slack.com/methods/users.profile.set

[Robert Thomas]

@Tozapid this is already possible:

err := slackUserClient.SetUserCustomFields(userId, map[string]slack.UserProfileCustomField{
	fieldId: {
		Value: fieldValue,
	},
})