fix #1354 -- copy .html source files as .src.html

This is the first fix described in #1354.  `.html` source files are now copied as
`.src.html`, verbatim.  This can be useful in some cases, eg. for debug purposes.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge pull request #1357 from getnikola/src.html

fix #1354 -- copy .html source files as .src.html