Add config file

This adds a config file for containerd configuration.  It is hard to
have structure data on cli flags and the config file should be used for
the majority of fields when configuring containerd.

There are still a few flags on the daemon that override config file
values but flags should take a back seat going forward and should be
kept at a minimum.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

vendor.conf

@@ -60,3 +60,5 @@ github.com/opencontainers/image-spec 0ff14aabcda3b2ee62621174f1b29fc157bdf335
 github.com/stevvooe/continuity 1530f13d23b34e2ccaf33881fefecc7e28e3577b
 # sync master as of 12/5/2016
 golang.org/x/sync 450f422ab23cf9881c94e2db30cac0eb1b7cf80c
+
+github.com/BurntSushi/toml      v0.2.0-21-g9906417