Remove language default and fix command flags

This commit stops using `node` as a default language due to it
conflicting with functions where the `fprocess` is set in the image.

Given the large numbers of languages having a default makes less sense
now.

It also removes some of the copy-paste/redundant/irrelevant flag
definitions that ended up in newly added subcommands.

Signed-off-by: John McCabe <john@johnmccabe.net>

MANUAL_CLI.md

@@ -10,11 +10,12 @@ So if you want to write in another language, just prepare a Dockerfile and build
 
 This will generate a Docker image for a Node.js function using the code in `/samples/info`.
 
-* The `faas-cli build` command can accept a `--lang` option of `python` or `node` and is `node` by default.
+* The `faas-cli build` command can accept a `--lang` option of `python`, `node`, `ruby`, `csharp`, `python3`, `go`, or `dockerfile`.
 
 ```
    $ faas-cli build \
       --image=alexellis2/node_info \
+      --lang= node \
       --name=node_info \
       --handler=./sample/node_info