Add charset checker to string rules

[andersonjoseph]

This PR add the following checkers:

• alpha (Return true if the value is a string with only alphabetic characters)

const schema = {
  x: {type: 'string', alpha: true}
}

• numeric (Return true if the value is a string with only numeric characters)

const schema = {
  x: {type: 'string', numeric: true}
}

• alphanum (Return true if the value is a string with only alphanumeric characters)

const schema = {
  x: {type: 'string', alphanum: true}
}

• alphadash (Return true if the value is a string with alphabetic characters and dashes)

const schema = {
  x: {type: 'string', alphadash: true}
}

[coveralls]

Coverage remained the same at 100.0% when pulling c527462 on andersonjoseph:master into f20da30 on icebob:master.

[icebob]

Perfect PR! Thank you.