Reimplementation (#1)

* refactor into separate modules

* update README

cli/parser.py

@@ -0,0 +1,81 @@
+"""Commandline argument parsing utilities."""
+
+from argparse import ArgumentParser
+
+
+def get_argument_parser():
+    """Sets up an argparse ArgumentParser and returns it."""
+
+    mainparser = ArgumentParser(
+        description="Debian ISO preseeding tool.",
+    )
+
+    # add mutually exclusive optional arguments to top-level parser
+    mainparser_group = mainparser.add_mutually_exclusive_group()
+    mainparser_group.add_argument(
+        "-o",
+        "--output-file",
+        action='store',
+        type=str,
+        dest='path_to_output_file',
+        metavar='OUTPUTFILE',
+        help="File as which the retrieved/generated file will be saved",
+    )
+    mainparser_group.add_argument(
+        "-O",
+        "--output-dir",
+        action='store',
+        type=str,
+        dest='path_to_output_dir',
+        metavar='OUTPUTDIR',
+        help="Directory into which the retrieved/generated file will be written",
+    )
+
+    # register subparsers for the 'get' and 'inject' subcommands
+    subparsers = mainparser.add_subparsers(
+        required=True,
+        title="Subcommands",
+        description="A choice of actions you want udib to take",
+        help="You must specify one of these",
+        dest="subparser_name",
+    )
+    subparser_get = subparsers.add_parser(
+        "get",
+        description="Retrieve an unmodified Debian ISO or example preseed file",
+    )
+    subparser_inject = subparsers.add_parser(
+        "inject",
+        description="Inject a preseed file into a Debian ISO",
+    )
+
+    # register arguments for the 'get' subcommand
+    subparser_get.add_argument(
+        "WHAT",
+        choices=['preseed-file-basic', 'preseed-file-full', 'iso'],
+        action='store',
+        type=str,
+        metavar='WHAT',
+        help="The type of file you want UDIB to retrieve. "
+             "Valid options are: 'preseed-file-basic', 'preseed-file-full' "
+             "or 'iso'.",
+    )
+
+    # register arguments for the 'inject' subcommand
+    subparser_inject.add_argument(
+        "PRESEEDFILE",
+        action='store',
+        type=str,
+        metavar='PRESEEDFILE',
+        help="Path to the preseed file you want to inject",
+    )
+    subparser_inject.add_argument(
+        "-i",
+        "--image-file",
+        action='store',
+        type=str,
+        dest='path_to_image_file',
+        metavar='IMAGEFILE',
+        help="Path to the ISO you want to modify",
+    )
+
+    return mainparser