Add ability to specify range of URLs to visit

* mu4e/mu4e-view.el (mu4e-view-go-to-url): Add the ability to visit
  multiple URLs by specifying a numeric range.
* mu4e/mu4e.texi (The message view: Keybindings): Document ability to
  visit multiple URLs.

`mu4e-view-go-to-url' now behaves like `mu4e-view-save-attachment'. You
can specify multiple URLs to visit at once by prefixing the visit
command by <C-u>; so C-u g asks you for a range of URLs to visit (for
example, 1 3-6 8). The range "‘a’" is a shortcut for all URLs.