Fix toggling of the numbered list prefix in Markdown tiddlers

[MaxGyver83]

Whenever you use a button/shortcut that calls the function prefix-lines.js, the line prefix is added when there was none or it's removed when it was already there. (Examples: bulleted list, numbered list, heading).

The only exception is numbered lists in Markdown tiddlers. When you press the according button multiple times, the prefixes add up:

1. 1. 1. item

The reason is that the prefix-lines function expects the prefix parameter to be a single character (which is true for all prefixes except for the Markdown numbered list prefix). This pull request makes prefix-lines work for multi-character prefixes, too.

[Jermolene]

Thanks @MaxGyver83 a minor comment

[Jermolene]

Thanks @MaxGyver83 there's a simple conflict which I would normally resolve as part of merging, but today I'm using GitHub's web interface, and so I think it might be safer if you could do it. Many thanks.

[MaxGyver83]

Thanks @MaxGyver83 there's a simple conflict which I would normally resolve as part of merging, but today I'm using GitHub's web interface, and so I think it might be safer if you could do it. Many thanks.

No problem. This was caused by my other pull request. I have rebased this branch onto master.

[Jermolene]

Thanks @MaxGyver83