Support both forms of aliased wiki links
Introduce a new boolean custom variable, markdown-wiki-link-alias-first.
When set to a non-nil value, markdown-mode will treat aliased wiki links
like [[link text|PageName]]. When set to nil, they will be treated as
[[PageName|link text]].
Note that the default behavior has been changed! The default behavior
now agrees with GitHub and other wikis which place the link text first.
To reinstate the previous behavior, which agreed with Instiki's behavior,
the new custom variable must be set to nil.