Vim: add vim-signature

vim-signature is a plugin to place, toggle and display marks. I have
been wanting to better utilize Vim's marks feature and I think that this
plugin might be helpful.

It defines all of the following mappings out of the box:

  m[a-zA-Z]    : Toggle mark
  m,           : Place the next available mark
  m.           : If no mark on line, place the next available mark.
                 Otherwise, remove (first) existing mark.
  m-           : Delete all marks from the current line
  m<Space>     : Delete all marks from the current buffer
  ]`           : Jump to next mark
  [`           : Jump to prev mark
  ]'           : Jump to start of next line containing a mark
  ['           : Jump to start of prev line containing a mark
  `]           : Jump by alphabetical order to next mark
  `[           : Jump by alphabetical order to prev mark
  ']           : Jump by alphabetical order to start of next line
                 containing a mark
  '[           : Jump by alphabetical order to start of prev line
                 containing a mark
  '?           : Open location list and display local marks

  m[0-9]       : Toggle the corresponding marker !@#$%^&*()
  m<S-[0-9]>   : Remove all markers of the same type
  ]-           : Jump to next line having same marker
  [-           : Jump to prev line having same marker
  ]=           : Jump to next line having any marker
  [=           : Jump to prev line having any marker
  m<BackSpace> : Remove all markers