Add ability to customize layout of Invoice

[soda3x]

Add ability to customize layout of Invoice, perhaps have a few templates that the user can select when generating the Invoice

[soda3x]

Moving this issue back to v0.5-alpha

[soda3x]

Deferred to v0.6-alpha

[soda3x]

For the future Java version, this will take shape in the form of allowing any key,value pair in a "theme".
If a "theme" specifies a field, it can be used on the invoice.
I think this will allow for great expandability and flexibility of the types of invoices you can create.

For example, the Persona might have an ABN field in the key,value store. The theme would just need to include {{abn}} in its HTML to utilise it.
This extends to every and all fields.