Fix helm plugin parse error on bash, and other improvements

[anguslees]

The following construct works on dash, but is a parse error on bash. Fixed in this change.

foo=$(cat <<EOF)
EOF

Fixes #17

Other improvements:

• Added long help description to kubectl plugin helm
• Fixed JSON quoting in upgrade subcommand
• Added workaround for kubernetes/kubernetes#53379
• Avoid strategic merge patch, which doesn't work with CRDs in k8s 1.8.