[MLton] tabs in library code (was: MLton library project)

[Stephen Weeks]

The consensus is clearly to allow tabs in personal libs if people want
them.  I've changed the pre-commit hook,

  /svnroot/hooks/pre-commit

to first see if the file matches a pattern in

  /svnroot/hooks/allow-tabs

and if so it omits the check for tabs.  That way, the default is to
disallow tabs, but we can easily allow them if people request.

Wesley, I've added "ca/terpstra" to allow-tabs.  Please try your
commit again, and let me know if it doesn't work.