v2.0.0

This release is tagged as major because of the following breaking changes in the public API:

• MetaHeader#initialize no longer accepts any arguments, parsing is delegated to a new MetaHeader#parse method (thus allowing reading from multiple source into the same instance)
• MetaHeader#validate now returns an empty array when no errors are found instead of nil
• MetaHeader.parse no longer accepts a MetaHeader object as argument
• The MetaHeader#strict attribute is replaced with a parameter in MetaHeader#validate
• The custom parser feature (via the MetaHeader::Parser abstract class) is removed

Other changes:

• Allow StringIO objects as input to MetaHeader#parse and MetaHeader.parse
• Fix "instance variable @parsers not initialized" warnings when no custom parsers are defined
• Remove invalid example for MetaHeader#alias in the documentation