Add ability to send attachments

[simonhughxyz]

This is more of a proof of concept or an outline of an idea, so don't merge this.

This adds a make_mime function to the send module which can generate a mime message from a mime lua table (a formatted table).

See send.example_mime to get an idea.

It also adds a test command you can run called :AttachTest which will generate a mime message in the current buffer from the send.example_mime table.

If you want to test sending a mime message, make the appropriate modifications to send.example_mime and then run :AttachTest in nvim, write the buffer to a temporary file and run the following command: cat FILENAME | msmtp -t.
You will need to have configured msmtp first.

These functions should help in adding attachments feature as I talked about in #5

Also, I am not to familiar with the mime message format, so I might have gotten some of it wrong, but I tested it with a pdf attachment and it works.

To get base64 encoding working for attachments, I used this module (https://github.com/iskolbin/lbase64)

[simonhughxyz]

I added the ability too add attachment from ComposeMail by using the <ctrl>g<ctrl>a mapping.
This will open a new window where you can add the path to each attachment one per line.
This needs to be the full path