Add support for nested tslint configuration files

[mantas6]

What problem does this feature solve?

Suppose I want to have a separate or albeit slightly different tslint.json config for my tests folder, currently it's not possible because config is only read from the project root directory.

What does the proposed API look like?

Nested tslint.json would be read and rules would be applied from the closest tslint.json file, starting from the file where linting is checked.

[haoqunjiang]

Now that TSLint is officially deprecated, TSLint support will be fully dropped in the next major version; as for the current major version, no more bug fixes or features will be implemented except for security reasons. So I'm closing this issue now.