Add completion for mech-dump

[mikkoi]

Part of WWW-Mechanize Perl distribution https://github.com/libwww-perl/WWW-Mechanize
WWW::Mechanize, or Mech for short, is a Perl module
for stateful programmatic web browsing, used for automating interaction with websites.

GitHub Issue https://github.com/scop/bash-completion/issues/474

[akinomyoga]

Please read CONTRIBUTING.md before submitting a PR.

The same completion seems to have been submitted to the upstream at libwww-perl/WWW-Mechanize#396, but the upstream hasn't yet rejected including the completion settings in their project.

This contains a hardcoded list of the options, though the options seem to be able to be extracted from the --help output.

The completion functions don't follow the naming convention at all.

The bundled completion doesn't need to anticipate that utilities bash-completion provides (such as _init_completion) are not available.