feat: Expand 'helm search'

[technosophos]

It has been suggested that we expand the scope of helm search, and add new similar tools, in an effort to improve findability of charts.

New output for Helm search

No longer show versions as a default in search output:

$ helm search nginx
stable/nginx
stable/nginx-reverse-proxy

Add a command: helm charts list

$ helm charts list
stable/alpine
stable/nginx
incubator/zookeeper

If helm search is called with no arguments, it returns the results of helm charts list

List all charts in a specific repository with helm charts list --repo stable

$ helm charts list --repo stable
stable/alpine
stable/nginx

Alias helm search --repo stable to the above.

List versions only when requested

$ helm search --versions nginx
stable/nginx      1.1.2
stable/nginx      1.2.3

[technosophos]

/cc @viglesiasce @migmartri @michelleN @prydonius @jackfrancis @adamreese

[technosophos]

As of #1229:

• The command helm search (no args) returns a list of all charts
• helm search stable/ (note the slash) returns all charts in the repo stable
• BUT charts are still listed by version number

[technosophos]

It has been requested that the helm charts * subcommands be moved to v2.1.0 so that we don't have to rush them right before Alpha.5. I think that all of the features listed above are now present (as of #1284). So when that gets merged, I will move the milestone on this to 2.1.0-Alpha.1 unless somebody objects.