Add a check for enforcing minimum length for variable names

Add a check for enforcing minimum length for variable names. A default
minimum length of three characters is applied to regular variables
(including function parameters). Loop counters and exception variables
have a minimum of two characters. Additionally, the 'i', 'j' and 'k'
are accepted as legacy values.

All three sizes, as well as the list of accepted legacy loop counter
names are configurable.