Factored html type out of boostbook.jam and in the process created a

modular system for registering new types.


[SVN r26484]

tools/build

@@ -1 +1 @@
-Subproject commit 0e7707514ef0ea2e5a262a80fe52ab1d840495ea
+Subproject commit 42f4871b1b5cb05a83603671874456f9304dec23