Read template alternatively also from STDIN

[Paebbels]

Currently a template needs to be provided as a file to command pyVersioning fillout <template file>. The command can already write to STDOUT or a given output file, but reading a template from STDIN is missing.

Alternatively, pyVersioning field <field> can be used, but it allows only a single field to be returned. By accepting a template from STDIN, multiple fields could be returned in a user-formatted order.