Allow custom hero images Fixes #379

[SrNetoChan]

Issue

Fixes #379

Description

Fixes issues that does not allow to set a custom hero for each post unless you have a bunch conditions met

It's based on a change proposed earlier but for some reason was deleted #661

Test Evidence

With this change, I was able to put the hero image in the assets folder and set the hero parameter in the post pointing to
/images/name_of_the_file.png

[hossainemruz]

Thank you @SrNetoChan

[eishundo]

This change breaks any existing posts that have the "hero" image in a folder location relative to the post category.

The following worked previously, but no longer works:

$ pwd
content/posts

$ /bin/ls
captive cubing hosting-history _index.md

$ bin/ls captive/index.md
captive/index.md

$ grep hero captive/index.md
hero: images/captive.gif

$ /bin/ls captive/images/captive.gif
captive/images/captive.gif

The question is, should ALL images be located in /images, or should post images be stored in a directory relative to the post itself?

[hossainemruz]

@eishundo This PR fixes the issue: #684

[eishundo]

@eishundo This PR fixes the issue: #684

Perfect! Confirmed to work under the original use case again. Many thanks.