Remove oauthtokencmd config and use oauthtoken

Instead of adding a new configuration variable, we reuse `oauthtoken` to
be able to specify a command by prefixing a !.

git-hub

@@ -343,14 +343,13 @@ class Config:
     def __init__(self):
         self.username = git_config('username', getpass.getuser())
         self.oauthtoken = git_config('oauthtoken')
-        self.oauthtokencmd = git_config('oauthtokencmd')
-        if self.oauthtokencmd is not None:
+        if self.oauthtoken is not None and self.oauthtoken.startswith('!'):
             try:
-                self.oauthtoken = subprocess.check_output(
-                        self.oauthtokencmd, shell=True).strip()
+                cmd = self.oauthtoken[1:]
+                self.oauthtoken = subprocess.check_output(cmd,
+                        shell=True).strip()
             except (OSError, subprocess.CalledProcessError) as e:
-                die("Failed to run oauthtokencmd `{}`: {}",
-                    self.oauthtokencmd, e)
+                die("Failed to run oauthtoken command `{}`: {}", cmd, e)
         self.upstream = git_config('upstream')
         if self.upstream and '/' not in self.upstream:
             die("Invalid hub.upstream configuration, '/' not found")

man.rst

@@ -479,9 +479,11 @@ from. These are the git config keys used:
   required, you shouldn't need to set this variable manually. Use the `setup`
   command instead.
 
-`hub.oauthtokencmd`
-  A command to retrieve the oauth token from a password store.
-  When set, `hub.oauthtoken` isn't needed.
+  If you don't want to store the token in plain text, you can also specify
+  a command by prefixing a `!`. The output of that command will be used as the
+  token. The command will be run with the default shell.
+
+  For example: `oauthtoken = !password-manager ~/my.db get github-oauth-token`.
 
 `hub.upstream` required
   Blessed repository used to get the issues from and make the pull requests to.