v1.1

Added:

• added VALUE and SINGLELINE built-in validators
• enhance documentation
• support Method objects being used as a custom validator

Changed:

• assume every tags are optional unless strict mode is enabled
• bump minimum supported Ruby version to v2.3
• report at most a single error per tag
• use single quotes in validation error messages

Fixed:

• cache the list of custom parsers for reduced CPU usage
• fix all Ruby warnings
• pass a boolean instead of an empty string to custom validators for boolean tags