Re-submit fix #33 version 2

As ian4hu pointed out in [#33](https://github.com/SublimeLinter/SublimeLinter-html-tidy/pull/SublimeLinter#33), Linter.which() called from within the HtmlTidy class definition resulted in an early invocation situation in which a fail test resulted for Linter's settings, _paths_ in particular, prior to any setting being cached.
Changes to fix this.

Author: Auz9

linter.py

@@ -18,9 +18,18 @@ class HtmlTidy(Linter):
     """Provides an interface to tidy."""
 
     syntax = 'html'
-    if Linter.which('tidy5'):
-        cmd = 'tidy5 -errors -quiet -utf8'
-    else:
-        cmd = 'tidy -errors -quiet -utf8'
+    executable = 'tidy'
+    version_args = '--version'
+    version_re = r'(?P<version>\d+\.\d+\.\d+)'
+    version_requirement = '>= 4.9'
     regex = r'^line (?P<line>\d+) column (?P<col>\d+) - (?:(?P<error>Error)|(?P<warning>Warning)): (?P<message>.+)'
     error_stream = util.STREAM_STDERR
+
+    def cmd(self):
+        """Return a tuple with the command line to execute."""
+        command = [self.executable_path, '-errors', '-quiet', '-utf8']
+        if Linter.which('tidy5'):
+            command[0] = Linter.which('tidy5')
+        else:
+            command[0] = Linter.which('tidy')
+        return command