_cli: Fix the behavior of the --desc flag (#153)

* _cli: Fix the behavior of the `--desc` flag

The `--desc` option's parameter is optional, and has the following semantics:

* No `--desc` whatsoever: `"auto"`
* Bare `--desc` (no parameter): `"on"`
* `--desc <choice>`: whatever choice is given

* CHANGELOG: record fix

* README: update `pip-audit --help`

CHANGELOG.md

@@ -14,6 +14,10 @@ All versions prior to 0.0.9 are untracked.
 
 ### Fixed
 
+* CLI: The `--desc` flag no longer requires a following argument. If passed
+  as a bare option, `--desc` is equivalent to `--desc on`
+  ([#153](https://github.com/trailofbits/pip-audit/pull/153))
+
 ### Removed
 
 ## [1.0.0] - 2021-12-1

README.md

@@ -27,7 +27,7 @@ python -m pip install pip-audit
 <!-- @begin-pip-audit-help@ -->
 ```
 usage: pip-audit [-h] [-V] [-l] [-r REQUIREMENTS] [-f FORMAT] [-s SERVICE]
-                 [-d] [-S] [--desc {on,off,auto}] [--cache-dir CACHE_DIR]
+                 [-d] [-S] [--desc [{on,off,auto}]] [--cache-dir CACHE_DIR]
                  [--progress-spinner {on,off}] [--timeout TIMEOUT]
 
 audit the Python environment for dependencies with known vulnerabilities
@@ -51,7 +51,8 @@ optional arguments:
                         auditing step (default: False)
   -S, --strict          fail the entire audit if dependency collection fails
                         on any dependency (default: False)
-  --desc {on,off,auto}  include a description for each vulnerability; `auto`
+  --desc [{on,off,auto}]
+                        include a description for each vulnerability; `auto`
                         defaults to `on` for the `json` format. This flag has
                         no effect on the `cyclonedx-json` or `cyclonedx-xml`
                         formats. (default: auto)
@@ -96,7 +97,7 @@ Flask 0.5     PYSEC-2018-66  0.12.3
 
 Audit dependencies including descriptions:
 ```
-$ pip-audit --desc on
+$ pip-audit --desc
 Found 2 known vulnerabilities in 1 packages
 Name  Version ID             Fix Versions Description
 ----  ------- -------------- ------------ --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

pip_audit/_cli.py

@@ -194,6 +194,8 @@ def audit() -> None:
         "--desc",
         type=VulnerabilityDescriptionChoice,
         choices=VulnerabilityDescriptionChoice,
+        nargs="?",
+        const=VulnerabilityDescriptionChoice.On,
         default=VulnerabilityDescriptionChoice.Auto,
         help="include a description for each vulnerability; "
         "`auto` defaults to `on` for the `json` format. This flag has no "