Allow for other tunnel types

[evilham]

I think this is related to #4 #3 albeit, I treated it more generically and actually did the thing:

• Added a type property to tunnels, that defaults to -L and accepts -D or -R
• If -D, RPort is optional and any setup value is ignored
• Its value is listed on ssh-tunnels table

If there is a chance of merging this and publishing to elpa, please reply and I'll add the documentation strings and create a Merge Request.

[death]

Sure, go ahead.