Add filter to encode/decode base64 strings

Problem/Motivation

In some cases is useful to process base64 strings, for example when implementing a front controller for some CI or PaaS services which handles config value as base64 encoded strings. Also can be used to encode metadata into HTML attributes wich in other way may break your markup.

Steps to reproduce

N/A

Proposed resolution

Implement 2 new filters base64_encode and base64_decode which will act on string values.

Remaining tasks

• Implement both filters on the "conversion" filters class
• Add some tests

User interface changes

N/A

API changes

Update the docs with new filters.

Data model changes

N/A