Add support for Mexico

[eprbell]

RP2 and DaLI have a programmable plugin architecture for countries, accounting methods, report generators and more. Adding support for a new country is a high-impact activity because it allows many new potential users to get access to RP2 and DaLI.

Here's how to add support for a new country:

1. add a new country plugin to RP2;
2. if the country requires accounting methods that aren't already supported in RP2, add the necessary accounting method plugins;
3. ensure any newly added accounting methods are reflected in the country plugin's get_default_accounting_method() and get_accounting_methods() methods;
4. optionally, new country-specific report generators can be added. The built-in report generators (i.e. rp2_full_report and open_positions) are automatically supported;
5. only for non-English speaking countries: translations for the built-in report generators and for any new report generator can be added;
6. ensure any newly added report generators are reflected in the country plugin's get_report_generators() method;
7. add the same country plugin to DaLI. The implementation on the DaLI side is trivial: it's just an entry point instantiating the RP2 country plugin and passing it to the main function;
8. update the documentation of RP2 and DaLI to reflect the new country, accounting methods (if any) and report generators (if any).

If you notice the country-specific infrastructure is missing a feature that is required to express taxes for the new country, open an issue to let the RP2 community know.