Add an option to conceal preformatted text markers

[gpanders]

This PR adds a feature allowing users to conceal code fence markers.

For example, if you have the following in your wiki:

Some text

{{{python
def hello_world():
    print("Hello world!")
}}}

then setting g:vimwiki_conceal_code_blocks would conceal the {{{python and }}} lines to produce:

Some text


def hello_world():
    print("Hello world!")

This can make things look a lot nicer and more polished in wikis with many code blocks.

Note that this only applies to blocks that are syntax highlighted. So plain blocks that use just {{{ }}} (with no syntax highlighting) will not be concealed.

[hq6]

Please fix the conflicts and force-push.

Also, what is the reasoning for handling syntax highlighted blocks differently than other pre-formatted text?

[gpanders]

No particular reason, I can modify it to include all pre-formatted text.

[gpanders]

Conflicts have been resolved. This is ready for merge.

[hq6]

I have manually merged this in e56c26c7ba892fc6ee073df0f2457d899d24d433 to have a cleaner history.