* add support for checkov for terraform
* add tests for checkov handler
* add basic linter config tests for checkov
* update supported tools and languages lists
* simplify ale_linters#terraform#checkov#Handle
* ensure "-o json --quiet" is always set for checkov
* add documentation for checkov including config options
* fix tests after changing handling of default options for checkov
* add checkov to list of tools in doc/ale.txt
