Added more XML files
- Fixed an issue with replacing <pb> tags in cases where they split
paragraphs of prose.
- Used a better regex for replacing multiple spaces.
Added XML files

Accidentally reverted trying to edit commit description. The following
XML files are now included in the data (10 new authors):
- Caesar, Catullus, Celsus, Columella, Lucan, Lucretius, Terence,
Valerius Flaccus, Virgil, and Vitruvius.