Image tag, plugin search

[ghost]

Added imgTag for path helpers.
Changed, added plugin directory environment variable,
this limiting full path scan for plugins.

[markbates]

Closing this in favor of #817

@backy-io Thanks for the PR. There were quite a few things that needed to happen to make this PR mergeable, so I tweaked them in #817.

A quick run down of what changed was the following:

• Fmt'd Code - there were issues with the code not being gofmt'd properly
• There was an unused import
• The idea of the BUFFALO_PLUGIN_PATH was great, but it broke backward compatibility, which for a point release is a no-no
• The splitting of the PATH was regressed. Windows separates on ; and unix on :
• All PRs need to be made against the development branch, as per the CONTRIBUTING.md file
• Finally, I cleaned up the assets helpers to reduce duplication and added tests.

Hopefully this helps for the next time. Thanks again fore PR!

[ghost]

Thx for comments, it was too early commit.