Custom validation functions support

[roo-oliv]

It may be useful to allow the caller to provide their own validation functions.

This PR adds support for overriding the default validation function globally or on a per-context basis.

[coveralls]

Coverage increased (+0.1%) to 93.109% when pulling 3e11be4 on allrod5:validate-function into 8944594 on rochacbruno:master.

[coveralls]

Coverage increased (+0.1%) to 93.109% when pulling 9fbcd07 on allrod5:validate-function into 8944594 on rochacbruno:master.

[coveralls]

Coverage increased (+0.1%) to 93.109% when pulling 9fbcd07 on allrod5:validate-function into 8944594 on rochacbruno:master.

[rochacbruno]

@allrod5 there are some conflicts because I merged your first PR first

[coveralls]

Coverage increased (+0.05%) to 93.093% when pulling b0a017c on allrod5:validate-function into c9c5e49 on rochacbruno:master.

[roo-oliv]

@rochacbruno Resolved'em

[coveralls]

Coverage increased (+0.05%) to 93.093% when pulling 2b1743a on allrod5:validate-function into c9c5e49 on rochacbruno:master.

[coveralls]

Coverage increased (+0.05%) to 93.093% when pulling 3de2c9d on allrod5:validate-function into c9c5e49 on rochacbruno:master.