Allow file/link extensions different than .html

Default functionality does not change, but there are now two new keys
available for the _site.yml file:
- link_ext: file extension to be added to the url links (instead of
default .html)
- file_ext: file extension to be added to the processed .md files
(instead of default .html)

Rationale:
In order to transparently replace a site that uses links without
extension ( i.e. www.test.com/account which actually accesses
account.html) it is necessary to keep links extension-less. For this
case (since .htaccess adds the .html to the requested urls) one uses
link_ext: '' and file_ext: '.html'.
However in other cases people may want to use the same for both (i.e.
'.htm' or '.php').