Draft structure for refactoring build process.
Build argument list for epub format.
Add initial epub build logic.
Migrate hooks to modules.
Add clean hook.
Add css concatenation hook.
Add --epub-stylesheet option to epub build.
Tidy abstract builder.
Write build output to log file.
Add epub3 build logic.
Add html build.
Add xhtml build.
Fix stylesheets for html and xhtml builds.
Add pdf build.
Fix epub argument paths.
Add txt build.
Add mobi build.
Tidy debug statements.
Implement build command.
Implement clean command.