Usability improvements

[asafh]

Hi @eberkund, I've made some usability improvements for this tool. I hope you find them useful as well. The changes:

1. Accepting the input files as parameters (with defaults where makes sense): keyfile, template, users csv
2. Allowing arbitrary fields in the template (and csv) by parsing it as a dictionary
3. Parsing the template file once and reusing it for all rendering
4. Allowing to generate signature html files as well (if some of your users use Outlook for instance)
5. (to be used together with 4) allowing to disable API calls (e.g. only want to check how the template is rendered for your users)

Cheers and thanks for sharing.