Audit and refactor the input and button components

[sebnitu]

Problem

The input component needs to be audited, cleaned up and have it's docs written.

Solution

This PR adds the missing input component README documentation and updates it's docs page to match. This also includes some major refactoring of the input component along with some minor button component consistency changes. Here are the majority of the changes made:

• Added calc-padding(padding, border-width)
• Added button styles to button component
• Added border width, style and color vars to button and input components
• Added example forms (example.html) to input component
• Refactored input component styles
• Removed input_color_invert modifier
• Refactored input_state_[value] modifier
• Refactored input_type_textarea modifier
• Refactor input box-shadow() mixin