Markdown documentation and default for --undefined.

README.md

@@ -94,8 +94,8 @@ Options:
     To import environment variables into the global scope, give it an empty string: `--import-env=`.
     (This will overwrite any existing variables!)
 * `-o outfile`: Write rendered template to a file
-* `--undefined`: Allow undefined variables to be used in templates (no error will be raised)
-
+* `--undefined={strict, normal, debug}`: Specify the behaviour of `j2cli` for undefined
+    variables. Refer to [Jinja2 docs][jinja2-undefined] for details.
 * `--filters filters.py`: Load custom Jinja2 filters and tests from a Python file.
     Will load all top-level functions and register them as filters.
     This option can be used multiple times to import several files.
@@ -137,4 +137,5 @@ j2cli is inspired by Matt Robenolt's [jinja2-cli][jinja2-cli].
 [ini]: https://en.wikipedia.org/wiki/INI_file
 [env]: https://en.wikipedia.org/wiki/Environment_variable#Unix
 [jinja2-cli]: https://github.com/mattrobenolt/jinja2-cli
+[jinja2-undefined]: https://jinja.palletsprojects.com/en/2.10.x/api/#undefined-types
 

j2cli/cli.py

@@ -133,7 +133,7 @@ def render_command(argv):
                         help='Load custom j2cli behavior from a Python file.')
     parser.add_argument('--no-compact', action='store_true', dest='no_compact',
                         help='Do not compact space around Jinja2 blocks.')
-    parser.add_argument('-U', '--undefined', action='store', dest='undefined',
+    parser.add_argument('-U', '--undefined', default='strict', dest='undefined',
                         choices=UNDEFINED.keys(),
                         help='Set the Junja2 beahaviour for undefined variables.)')
     parser.add_argument('-I', '--ignore-missing', action='store_true',