Changes to embeds cog

[ItsDrike]

The new embeds cog is good, but it could use some further edits:

• An option to add text to description instead of only setting it. (This also applies to fields)
• An option to set the text of a non-embed message (can be helpful for mentions), this message should be posted before the embed itself
• Fix footer mentioning: Mentioning user/channel in footer produces a <#[channel id]>/<@![user_id]> instead of normal mention, while it is impossible to directly mention a channel in the footer, it should use plaintext for mentions @mention or #channel even though it will not produce a clickable mention, it is much more readable compared to the current formatting.

[ItsDrike]

The embed message sent to #mod-log should also change from: To simply: Which is much less cluttered, there is no point in showing the full url

Implementation There are more markdown features in embeds compared to normal messages. Something like [url](https://someurl.com) is fully supported in embeds.

[ItsDrike]

Another thing which should be fixed is the message after !embedbuild command referring to use !help Embed for more info is wrong as !help Embed is not recognized, it should actually be !help embed