Configuration for changing ignore comment text

[dsherret]

Right now ignore comments have the text "dprint-ignore", "dprint-ignore-start" and "dprint-ignore-end", but this should be configurable.

It is being really lazy now and using regex, but that should be removed and replaced with character comparisons.

[bartlomieju]

@dsherret I can look into this issue, would it be satisfactory if I accepted the directives as part of the config and then store proper regexes on Context?

[dsherret]

@bartlomieju sure!