add remove-watch and use-timeout attribute to the markdown directive

Many angular watches are of the markdown directive, i've added two attributes which could improve step-by-step the performance.

For example if we use-timeout, we could delay the markdown parsing for some ms, and not start every markdown parsing instantly.

Also not every markdown needs to watch for changes, here comes the remove-watch attribute to help, for example this could be used on the chat messages 

Ping @paglias @crookedneighbor @lefnire  Any other ideas on this directive which I should implement? If not I'll merge this in 1-2 days :)