Add formatSIRET, formatSIREN, formatVAT methods + tests

[WaldoJeffers]

Add the following methods:

• formatSIRET
• formatSIREN
• formatVAT

These methods internally use isXXXX to check if the input value is valid, and throw an error otherwise.

The regexps could probably be optimized, but I think this will do fine. Also, it might be interesting to have a formatSIRETorSIREN kind of method, because most programs accept both SIRET & SIREN from users.

I also added tests 😄 Feel free to give me some feedback if you'd like something done differently

Cheers

[coveralls]

Coverage remained the same at 100.0% when pulling 24e2d24 on WaldoJeffers:master into cd22d5b on jbdemonte:master.