New feature: show line number correspondences

Option --linemap on uncompile show how original source-code line numbers
map to uncompiled source lines

uncompyle6/linenumbers.py

@@ -0,0 +1,14 @@
+from xdis.load import load_file, load_module
+from xdis.bytecode import findlinestarts, offset2line
+
+def line_number_mapping(pyc_filename, src_filename):
+    (version, timestamp, magic_int, code_obj1, is_pypy,
+     source_size) = load_module(pyc_filename)
+    try:
+        code_obj2 = load_file(src_filename)
+    except SyntaxError as e:
+        return str(e)
+
+    linestarts_orig = findlinestarts(code_obj1)
+    linestarts_uncompiled = list(findlinestarts(code_obj2))
+    return [[line, offset2line(offset, linestarts_uncompiled)] for offset, line in linestarts_orig]