Allow tags in markdown and mediawiki syntax as well

Ref #85

syntax/vimwiki_markdown.vim

@@ -88,3 +88,4 @@ let g:vimwiki_rxMathStart = '\$\$'
 let g:vimwiki_rxMathEnd = '\$\$'
 
 let g:vimwiki_rxComment = '^\s*%%.*$'
+let g:vimwiki_rxTags = '\%(^\|\s\)\@<=:\%([^:[:space:]]\+:\)\+\%(\s\|$\)\@='

syntax/vimwiki_media.vim

@@ -69,3 +69,4 @@ let g:vimwiki_rxMathStart = '{{\$'
 let g:vimwiki_rxMathEnd = '}}\$'
 
 let g:vimwiki_rxComment = '^\s*%%.*$'
+let g:vimwiki_rxTags = '\%(^\|\s\)\@<=:\%([^:[:space:]]\+:\)\+\%(\s\|$\)\@='