add formatter interface

PyTeX/formatting/formatter.py

@@ -0,0 +1,25 @@
+from typing import List, Optional, Dict, Tuple
+from pathlib import Path
+
+
+class Formatter:
+    """
+    A formatter is bound to a specific input file with some
+    building configuration.
+    """
+    def format(self, build_dir: Path) -> Optional[List[Tuple[str, Dict]]]:
+        """
+        :param build_dir: Directory where output files are written to
+        :return: When configuration files are needed for a future
+        build of the output files, a list of the file names and their
+        needed configurations. Else None.
+        """
+        pass
+
+    @property
+    def output_files(self) -> List[str]:
+        """
+
+        :return: List of files that will be built when the formatter is invoked
+        """
+        pass