Add maxLength prop

[ChrisUser]

Description

Proposed in #12

• Add maxLength prop
• Add charsCounterClassName style prop
• Update methods
• Update docs

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• I'm sure that this PR is not a duplicate
• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes