subcommands: merge and tidy

[iomz]

The current version of bibfuse imports bibtex entries from multiple .bib files.
However, the .bib file tends to become "too fat" when writing manuscripts – having many references that are not cited in the .tex files.
Therefore, I propose to create two new subcommands.

merge: to perform the current import/merge/output task producing a .db file containing all the entries.
tidy: takes a .db file and multiple .tex files, and generates a new .bib file that only contains the references that appear in the .tex files.