config: add spam option for extract_text

config: add spam option for extract_text

toggling the configuration options for the ExtractText SA plugin (see
[0]).

The config is copied from the module itself, the informational headers
were not added, as I don't see too much gain, apart from verifying
that the plugin is working.

the external dependencies for the plugin to work are added as
Recommends, as it is a possible config to not have them installed and
simply disable the option

[0] https://metacpan.org/pod/Mail::SpamAssassin::Plugin::ExtractText
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>