New commands: subed-merge-with-next, subed-merge-with-previous

[sachac]

• subed/subed.el (subed-mode-map): Bind M-m to subed-merge-with-next. Bind M-M to subed-merge-with-previous (subed--generic-function-suffixes): Add generic merge-with-next.
• subed/subed-common.el (subed-merge-with-previous): New command.
• subed/subed-srt.el (subed-srt--merge-with-next): New command.
• subed/subed-vtt.el (subed-vtt--merge-with-next): New command.

All right, this should be just the diff for merge with next/previous that can be applied to your sacha branch. =)

[rndusr]

That merged like a charm. And so do the subtitles. Thanks!