Tweak to +/- wikitext escaping rule in td-cells.

- When in a td-cell +/- need not be escaped if the td-char is
  not a single "|" char. Ex: "||+1.5"

- Fixes RTing of snippet "{|\n||+1.7\n|}" extracted from
  en:100 Metre Dash

- No change in parser tests. TODO: Add more.

Change-Id: Id069c99ce037ebb62bc4f2555c4d850920f4acc2