Add support for customising the DayCell

[kamyar]

What types of changes does your code introduce?

• New feature (non-breaking change which adds functionality)

Description

Adds support for passing a function to customize DayCell content.

Related Issue: #242

[brunolucena]

+1

[mkg0]

LGTM except for one thing

[onionhammer]

Looks great!

[charithAmila]

Looks good

[HugoLiconV]

Hi everyone! Wouldn't be useful to implement something similar to this but for the whole button, not just for the content? So instead of doing this:

<button
// ...buttonProps
>
  {dayContentRenderer?.(this.props.day) || (
    <span>{format(this.props.day, this.props.dayDisplayFormat)}</span>
  )}
</button>

We'll be doing this:

if (dayContentRenderer) {
  return dayContentRenderer({  
    ...buttonProps
  });
}
return (
  <button
     // ...buttonProps
  >
    <span>{format(this.props.day, this.props.dayDisplayFormat)}</span>
  </button>
)

What do you think?