Support string API

[dvershinin]

Proposed changes

Currently you have to create a temporary file just so you can call crossplane.parse on it, which seems needlessly awkward.

Gixy analyzer tool would particularly benefit from this, as it reads config files first (from stdin often), then has to create temp files because crossplane does not yet support a string API.

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork