More Prolog recipes for extending rules

[ihh]

The ability to add Prolog clauses to rules is the unsung feature of biomake: potentially quite powerful, but lacking clear use cases & recipes.

How about a few examples in the README? Using re_match in the rule, as you can now do because biomake includes pcre, is one quick & powerful way to use this feature. We could put this into the README along with other examples e.g.

• rule only fires if TARGET file is writable
• rule only fires if TARGET file is older than one week
• rule only fires if all DEPS files are under 1Mb in size
• results go in directory X unless source file is Y, in which case they go into Z (use prolog/endprolog with a little prolog database that specifies the results dir)