Override titlepage template epub

[Geobert]

This allows to override epub.titlepage.xhtml and fixes a bug in the output message of the progress bar (the file was correctly generated).

[Geobert]

Also a question:

warning: fields `short_message` and `issue_type` are never read
  --> src/lib\grammar_check.rs:38:9
   |
34 | struct GrammarError {
   |        ------------ fields in this struct
...
38 |     pub short_message: Option<String>,
   |         ^^^^^^^^^^^^^
39 |     pub issue_type: Option<String>,
   |         ^^^^^^^^^^
   |

Are these fields used somewhere else (when using crowbook as a lib)?

[crowdagger]

Thanks for the PR, sorry for the enormous delay. And no those fields are not use, I really need to do some cleaning one of these days :)