Get extended entry info (<!--more-->).There should not be any space after the second dash and before the word
'more'. There can be text or space(s) after the word 'more', but won't be
The returned array has 'main', 'extended', and 'more_text' keys. Main has the text before
the `<!--more-->`. The 'extended' key has the content after the
`<!--more-->` comment. The 'more_text' key has the custom "Read More" text.