Update the parser with the new problem type "Clustering"

[sagnik1511]

What you have to do -
1. Update the parser's problem type definitions.
2. Update the tab_automl.utils.misc.validate_parse_variable as it was prepared to check only the problem types of classification and regression.
3. The target variable parser should have a default value None as the clustering problem won't allow any target variable, but keep in mind if the problem type is some supervised technique, then the target_feature should be checked inside .tab_automl.utils.misc.validate_parse_variable function.
4. Also update the README.md where it specifies the problem types.

Follow contributing guidelines on README.md