Small edits: moved variables to metadata.yml to use in all output

[davidverweij]

In the existing implementation the code for the custom front page for the .pdf and the HTML export was written separately (not using joint variables). I've made some small edits:

• moved the title, author, etc. to a metadata.yml file,
• moved the content of 01_title_page.md to the template.tex
• removed redundant code in the .html and .tex templates
• rewrote those templates to use the variables in the metadata.yml file
• added an optional subtitle variable

The resulting pdf and HTML is identical - except for the addition of a subtitle.

[davidverweij]

Let me know if you want me to merge a new master into this PR, if you merge any of the other PR's - like #94 . Cheers!

[dendrondal]

I like the idea of splitting up the YAML. Would definitely wait for #94 to merge, just in case XNOS breaks something.

[tompollard]

Thanks @davidverweij, looks good! The pull request needs a rebase after #94 was merged. Would you like to do this, or shall I?

[davidverweij]

Apologies on the extreme delay! Holidays, deadlines and being new to rebasing caused me to put this down my to-do list. Anyway, I did a rebase and ran the code to test - all seems good from my end.

[tompollard]

No apologies needed! Looks great, thanks for doing this :)