Fixed conflicting CLI flags

[tlockney]

The -a used for the auth credentials and the shortened form of the --all flags were in conflict on the fetch command. To be consistent with other -to-sqlite libraries in the Dogsheep ecosystem, I removed the shortened form of the --all flag.

[simonw]

I thought this might need a documentation update but --all is already covered: https://github.com/dogsheep/pocket-to-sqlite/blob/0.2.1/README.md