[Feature] Download common tools

[alexellis]

Download common tools

Expected Behaviour

Download tools like kubectl, kubectx, k9, faas-cli and so forth.

Current Behaviour

helm 2 / 3 and Linkerd are downloaded into ~/.arkade when certain apps are installed.

Possible Solution

Take the approach from the faas-cli -> https://github.com/openfaas/faas-cli/blob/30b7cec9634c708679cf5b4d2884cf597b431401/commands/cloud.go#L155

Steps to Reproduce (for bugs)

Probably something like get should be used instead of install so that there is a separation of concerns between apps and tools.

arkade get kubectx

# Seems a bit long to type
arkade download kubectl

[alexellis]

Fixed in: #97

This initial version of arkade get works with binaries on release download URLs, folders that are zipped, or individual files which are tar/gz'd does not work yet and will require additional work.