Add hyphens in place of speaker names in SDH stripped subtitles

[vevv]

Some SDH subtitles prefix one sentence with a speaker name, and the second one with nothing (or both with a speaker name). After stripping, those look like a single sentence, rather two separate lines. Subtitle Edit already does this, see example below:

Source:

1
00:07:24,744 --> 00:07:27,950
MANTIS:
What are you going to do?
Me? He's your brother.

2
00:08:58,109 --> 00:08:59,138
MAN: What was that?
WOMAN: What was that?

3
00:08:59,274 --> 00:09:00,941
NEBULA: What the hell?
BOY: Oh, my God!

subby:

1
00:07:24,744 --> 00:07:27,950
What are you going to do?
Me? He's your brother.

2
00:08:58,109 --> 00:08:59,138
What was that?
What was that?

3
00:08:59,274 --> 00:09:00,941
What the hell?
Oh, my God!

Subtitle Edit:

1
00:07:24,744 --> 00:07:27,950
- What are you going to do?
- Me? He's your brother.

2
00:08:58,109 --> 00:08:59,138
- What was that?
- What was that?

3
00:08:59,274 --> 00:09:00,941
- What the hell?
- Oh, my God!