Removes restrictions in viewport names

[nkakouros]

In the regexp that handle viewports, there is the restriction on header names to not include the ] or } characters. I couldn't see why this restriction is in place as the | is what actually matters.

This PR removes the restriction. It means to address #256 .

[liskin]

The restriction is there because vimwiki links may contain |: https://github.com/vimwiki/vimwiki/blob/619f04f89861c58e5a6415a4f83847752928252d/doc/vimwiki.txt#L996

It's definitely possible to write a regexp that handles this, but it's going to be more complex than the current one. Several options come to my mind:

1. keep the [^=\|\[\{]* and add an alternative that recognizes [[ and then eats everything until ]]
2. use a negative look-ahead assertion (\@!) to make sure that the | isn't followed by ]]
3. require spaces around the | as vimwiki links are unlikely to contain spaces around their |

The third option is definitely the easiest, but I have no idea if that would be acceptable to @tbabej. It's also the one that is most likely to break some users' usecases.

Either way, now that the tests are working and fast, it would be a good idea to add a few tests covering the various scenarios.

[nkakouros]

I went with the simplest approach.

[nkakouros]

I also fixed markdown links not being followed. This is ready to merge if you have no comments.

[nkakouros]

@tbabej Friendly reminder that this seems ready to merge.