Per-window jumplists and some functions for jumplist manipulation.
Use marks instead of points in jumplist.
jumpBackE and jumpForwardE now jump across buffers.
Update jumplist when doing certain motions and opening new files.
Search commands now update jumplist.
Ask travis to use alex --debug.
Adapt to new alex API, part 1

Build with alex >= 3.0.3 still fails because lexers are somehow
generated using "-nopred" template, which is apparently not what we want.