Support Containerfile

[jfcherng]

Syntax: https://github.com/keith-hall/Containerfile-sublime-syntax

It seems to provide better syntax for dockerfile but it renames to Containerfile (including scope)
keith-hall/Containerfile-sublime-syntax@ce75be1

Proposed Change

In plugin settings,

-    "selector": "source.dockerfile"
+    "selector": "source.dockerfile | source.containerfile"

[rchl]

It should also be considered to either replace the existing recommendation or mention both syntaxes in the Readme