2shared (upload): add missing document support

Thanks to nick@freexrates.com for reporting.
uploaded.net (premium download): fix

Thanks to fjvazquezaraujo for reporting.
plowdown: --run-before/--run-after continue if script exits with non zero status

An error message is printed but continue normally.
Except for --run-before, status 2 ($ERR_NOMODULE) will skip current link.
docs: add --run-before/--run-after

Changes:
- bash completion updated
- drop --exec
plowdown: --run-before, ignore delay (same previous hoster) when previous link has been skipped