Add back mechanism for cross-tool hyperlinks.

The exported HTML pages again can contain hyperlinks to other tools.
Instead of replicating the old functionality, made the links of the form
[gmx-distance] instead of recognizing plain text words.  This requires
all the output formats to be aware of them (not a big deal in the
current code layout), and makes it possible to define the appearance of
the links centrally instead of relying on all links being wrapped in,
e.g., [TT]...[tt].  The new link syntax is similar to Markdown implicit
links (at least Doxygen accepts them without the second []).

Also added support for [THISMODULE] tag in the help text, which expands
to the name of the current module, again allowing the appearance to be
defined centrally.

Only changed the help text for gmx-mindist to take advantage of the new
mechanism as a proof-of-concept.  Bulk work for other tools is better
done as a separate commit once the general approach is accepted.

Part of #685 and #1242.

Change-Id: Ibd263cc2c131dc18d2a5d9046fecc6b1c08734f9