Add support for options to control documentation output and search path

[AndyKilpatrick]

I've added the following options
-p : too pass a search path to the command line tool, useful when building and testing against helm charts in different locations

• s: values.yaml fields with no descriptive comments are omitted from the documentation produced. Reasoning: We have large umbrella helm charts; the values.yaml includes third party subcharts and some internal "development" fields; we need control over which fields are documented.

• c: prevents objects / lists being documented in the default field. We have deep nested objects in values.yaml. We want to be able to add documentation string for a top level object (and some of its children) without the entire (large, complex) object visible in the default column in the documentation. With this option, a blank default field ("") is produced

The long names of these fields are not easy to pick - alternative suggestions welcome.

Added some tests for the new options

[norwoodj]

Apologies @AndyKilpatrick, I've been having medical issues which have prevented me from doing a lot of typing the last few months. As a result, I've basically had to ignore this repository for a while. I will get to reviewing this PR in the next few days. Looks like a very sensible change, so I don't think there should be any problems with it.