mu4e-views

`mu4e' is great, but viewing of html emails is suboptimal.  This packages
enables the user to choose how to view emails.  It's main use case is to view
emails using an xwidgets window, but the user provided viewing methods are
also supported.

Also provides methods for user defined viewing methods to access content
extracted from an email, e.g., urls or attachments.  This makes \it easier to
build new views.