New option -U, --context-lines to solve incorrect multiline syntax highlighting

[dandavison]

Fixes: #117 #316 #509 #577 #1091 #886

Thanks to the aptly-named @ideasman42 in #1091.

  -U, --context-lines <N>
          Display at most N context lines around each change.

          When the input diff has more context lines than N, delta will use all of them for syntax
          highlighting but only display N. This can fix incorrect syntax highlighting around
          multiline constructs (multiline strings, block comments, etc). Use this in conjunction
          with git's -U option, e.g. `git diff -U9999`.