notmuch.el: Display authors in a column separate from subject.

notmuch.el: Display authors in a column separate from subject.

This makes it much easier to actually read the subject lines.

The user can set notmuch-search-authors-width to control the width of
the column.

Two possible ideas for improving this support further:

  1. Make the excess authors invisible instead of removing them from
     the buffer, (which means that isearch could still find them).

  2. Have the user variable control a percentage of the window width
  rather than being a fixed number of columns.