start: Change --config and --runtime to --bundle

Personally I prefer a single config file [1].  I want folks to be able
to pipe their config into the 'funC start' command (e.g. via a
/dev/fd/3 pseudo-filesystem path) [2], and I have a working example
that supports this without difficulty [3].  But since [4] landed on
2015-11-16, runC has replaced their --config-file and --runtime-file
flags with --bundle, and the current goal of this repository is
"keeping as much similarity with the existing runC command-line as
possible", not "makes sense to Trevor" ;).

[1]: https://groups.google.com/a/opencontainers.org/forum/#!topic/dev/0QbyJDM9fWY
     Subject: Single, unified config file (i.e. rolling back specs#88)
     Date: Wed, 4 Nov 2015 09:53:20 -0800
     Message-ID: <20151104175320.GC24652@odin.tremily.us>
[2]: opencontainers/runc#202
[3]: https://github.com/wking/ccon/tree/8ab5b535b5eca1a62e12b5e865735e24f1e1666d#configuration
[4]: opencontainers/runc#373

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime.md

@@ -48,8 +48,7 @@ Start a container from a bundle directory.
 * *Options*
   * *`--id <ID>`* Set the container ID when creating or joining a container.
     If not set, the runtime is free to pick any ID that is not already in use.
-  * *`--config <PATH>`* Override `config.json` with an alternative path.  The path may not support seeking (e.g. `/dev/fd/3`).
-  * *`--runtime <PATH>`* Override `runtime.json` with an alternative path.  The path may not support seeking (e.g. `/dev/fd/3`).
+  * *`--bundle <PATH>`* Override the current working directory as the bundle path.
 * *Standard streams:* The runtime must attach its standard streams directly to the application process without inspection.
 * *Environment variables*
   * *`LISTEN_FDS`:* The number of file descriptors passed.