Add antlr4-test task to run ANTLR4 TestRig from within Leiningen

lein-antlr4 plugin can now be used to run TestRig tool on generated grammar. Command-line args are passed directly to the tool. Generated classes need to be compiled first.

> lein antlr4-test
java org.antlr.v4.runtime.misc.TestRig GrammarName startRuleName
  [-tokens] [-tree] [-gui] [-ps file.ps] [-encoding encodingname]
  [-trace] [-diagnostics] [-SLL]
  [input-filename(s)]
Use startRuleName='tokens' if GrammarName is a lexer grammar.
Omitting input-filename makes rig read from stdin.