fix(highlight): increase syntax highlighter config priority

[glevco]

This PR increases the highlighter config priority over the rgb_colors config. It resolves #337.

Currently, enabling the syntect-highlighter feature is not enough to actually enable syntax highlighting, because the default rgb_colors config disallows it (more info and current workarounds in #337). It would be desirable if users could use syntax highlighting simply by adding the syntect-highlighter feature.

Also, the current behavior is not consistent. For instance, when a custom highlighter is set (instead of syntect's), rgb_colors is already disregarded. When a custom highlighter is set and syntect-highlighter is disabled, even the color config is disregarded (it shouldn't be).

I kept 2 commits that may or may not be reviewed separately:

1. The first commit just adds new tests to assert the current (unexpected) behavior. It makes no changes in behavior.
2. The second commit performs the actual fix/refactor.

I did this so the change in behavior is explicit in the diff between the commits, by observing how the tests were changed. By the way, this is a breaking change since default behavior is changed.