Add ability to list only feeds with CLI

@csantanapr commented on [Mon Jun 06 2016](https://github.com/apache/incubator-openwhisk/issues/575)

A feed is an action with a feed=true annotation.
We can enhance the `wsk action list` to filter for feeds like `wsk action list --feed`

Another alternative is to treat feed as first class objects, and have a `feed` command like `wsk feed list`

Making a the new command:

```
$ wsk action -h
usage: wsk feed [-h] {create,update,get,delete,list} ...

optional arguments:
-h, --help show this help message and exit

available commands:
{create,update,invoke,get,delete,list}
create create new action
update update an existing action
get get action
delete delete action
list list all actions
```

cc @mbehrendt

---

@mbehrendt commented on [Mon Jun 06 2016](https://github.com/apache/incubator-openwhisk/issues/575#issuecomment-223978644)

to list feeds, a developer using whisk should _never_ have to know that feeds are implemented as actions (actions are just impl choices). an event provider creating feeds has to know that, but not a regular user.

i'd suggest to either expose feeds as a first class entity or allow listing them with a parameter to `wsk package list`, eg `wsk package list -feeds`

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@csantanapr commented on [Mon Jun 06 2016](https://github.com/apache/incubator-openwhisk/issues/575#issuecomment-224060077)

Hum `wsk package list` feels wrong, this command it's about listing only packages, `wsk package get $name` or `wsk package get $name --summary` it's about list the package content.

This will get you the summary of the specified package like:

```
$ wsk package get myGit --summary
package /csantana@us.ibm.com_dev/myGit
feed /csantana@us.ibm.com_dev/myGit/webhook: Creates a webhook on github to be notified on selected changes
```

What about `wsk list` listing feeds?

Today it `wsk list` prints a summary for `packages` `actions` `triggers` and `rules` but it doesn't list `feeds`

for example the `feed` `/csantana@us.ibm.com_dev/myGit/webhook`
Can be listed very easily with `wsk list` like this:

```
$ wsk list
```

entities in namespace: csantana@us.ibm.com_dev
**packages**
/csantana@us.ibm.com_dev/myGit private binding
**actions**
/csantana@us.ibm.com_dev/echo private
**triggers**
/csantana@us.ibm.com_dev/myGitTrigger private
**rules**
/csantana@us.ibm.com_dev/ifGitThenEcho private
**feeds**
/csantana@us.ibm.com_dev/myGit/webhook private

---

@markusthoemmes commented on [Tue Jun 07 2016](https://github.com/apache/incubator-openwhisk/issues/575#issuecomment-224281346)

If `wsk list` lists feeds, I as a user expect to be able to do something like `wsk feed get` as I can do that with all the other entities. That'ld strenghten @mbehrendt 's point of having feeds as first-class entities, at least on CLI level.

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@csantanapr commented on [Tue Jun 07 2016](https://github.com/apache/incubator-openwhisk/issues/575#issuecomment-224342804)

I'm in agreement that it should be first class citizen and on top of that also be listed in `wsk list`

The bigger question is should the REST API supports them as first class citizens not just the CLI?

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@rabbah commented on [Tue Jun 07 2016](https://github.com/apache/incubator-openwhisk/issues/575#issuecomment-224343620)

no.

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@csantanapr commented on [Tue Jun 07 2016](https://github.com/apache/incubator-openwhisk/issues/575#issuecomment-224402666)

our emoji voting 😄 in https://github.com/openwhisk/openwhisk/issues/575#issuecomment-224343620 seems to indicate that making them look like first class will be handle client side (i.e. CLI, UI, etc..)