Support for library charts

**Is this a request for help?**:

Probably

**Is this a BUG REPORT or FEATURE REQUEST?** (choose one):

This is a **FEATURE REQUEST**. I would like to be able to **test library charts** to
some degree using the chart-testing automation.

This will help with maintaining library charts since it could make it easier to change them if they are properly tested.

**Version of Helm and Kubernetes**:

"latest"

**What happened**:

I created a library chart on a repo with the chart-testing GitHub Action enabled.

**What you expected to happen**:

That chart-testing does some sort or more or less meaningful tests on my library chart.

**How to reproduce it** (as minimally and precisely as possible):

Run the install tests of chart-testing against a `charts/` folder containing a library chart.

**Anything else we need to know**:

I think there are a few reasonable ideas on how to tackle this but wanted to get some input:
* this could be out of scope for chart-testing and I should ignore my library. If this is the way to go I should try to always combine any changes to the library chart with changes to an example chart that uses the library (this might result in some sorts of dependency-hell when it comes to managing Chart.lock files in PRs)
* chart-testing could support a concrete example of the library in the `ci/` folder and use that for testing
* chart-testing could only support linting library charts and not exit with and simply not attempt to install a library chart (i can already do this by ignoring my library chart)