Added support for 'line-no-highlight' class

Added the 'line-no-highlight' class to lines that are within an embed containing highlighting, making it possible to specifically style non-highlighted lines. The class doesn't apply to embeds that have no highlighting.
It's been a while since I worked on this, and @bradyvercher might correct me, but the opening condition of:

`if ( isset( $highlight[ $key + 1 ] ) ) {`

...says that "if highlighting is to be applied to this line, then...", which means the else translates to "no highlighting should be applied to this line". 

Hang on. I'm going on the assumption that you're after the following.

----
Some highlighting:

.line .line-no-highlight
.line .line-no-highlight
.line .line-no-highlight
.line .line-highlight
.line .line-highlight
.line .line-no-highlight

No highlighting:

.line .line-no-highlight
.line .line-no-highlight
.line .line-no-highlight
.line .line-no-highlight
.line .line-no-highlight
.line .line-no-highlight

-----

But are you actually after:

----

Some highlighting:

.line .line-no-highlight
.line .line-no-highlight
.line .line-no-highlight
.line .line-highlight
.line .line-highlight
.line .line-no-highlight

No highlighting:

.line
.line
.line
.line
.line
.line

----

? Only adding the no-highlighting if there is at least one line which does have highlighting? In which case, what you've got would be correct.