Add coordinates to the ASCII-board

[micha003]

It would be nice, if the ASCII-board in the console would get coordinates on the sides (left and bottom) to keep an overview, where
the pieces are. It is a benefit for making moves faster (you don't have to think about it, where the pieces are).

[niklasf]

The closest that exists currently is:

>>> print(chess.Board().unicode(borders=True, empty_square=" "))
  -----------------
8 |♜|♞|♝|♛|♚|♝|♞|♜|
  -----------------
7 |♟|♟|♟|♟|♟|♟|♟|♟|
  -----------------
6 | | | | | | | | |
  -----------------
5 | | | | | | | | |
  -----------------
4 | | | | | | | | |
  -----------------
3 | | | | | | | | |
  -----------------
2 |♙|♙|♙|♙|♙|♙|♙|♙|
  -----------------
1 |♖|♘|♗|♕|♔|♗|♘|♖|
  -----------------
   a b c d e f g h