Support exporting to ASCII/unicode diagrams

[waldyrious]

ASCII (as proposed by David Potsiadlo's Fretboard):

 x  ^ ^ 
 ||||o| 
 ||o||| 
 |o|||| 
 |||||| 

Unicode (as proposed by David R. Arnold's OpenGuitar):

  C7       D       C       G   
  x   x    o     x  o o    ooo 
 ╒╤╤╤╤╕  ╒╤╤╤╤╕  ╒╤╤╤╤╕  ╒╤╤╤╤╕
 ││││1│  ││││││  ││││1│  ││││││
 ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤
 ││2│││  │││1│2  ││2│││  │1││││
 ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤
 3││4││  ││││3│  │3││││  2││││3
 ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤

Alternative, using some sort of bullets instead of finger numbers:

  C7       D       C       G   
  x   x    o     x  o o    ooo 
 ╒╤╤╤╤╕  ╒╤╤╤╤╕  ╒╤╤╤╤╕  ╒╤╤╤╤╕
 ││││•│  ││││││  ││││⧫│  ││││││
 ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤
 ││•│││  │││●│●  ││⧫│││  │◆││││
 ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤
 •││•││  ││││●│  │⧫││││  ◆││││◆
 ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤  ├┼┼┼┼┤

(ideally we should be able to combine both approaches using ➊➋➌➍, but these characters break the lining up because they're slightly wider)