porter mixin install --feed-url command

[carolynvs]

$ porter mixin install helm --feed-url https://porter.sh/mixins/atom.xml
installing latest

• feed categories = list of mixins published in the feed
• category term = mixin name
• content = mixin version
• link = download url (must still follow the filename convention)

Example Atom Feed

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <id>https://porter.sh/mixins</id>
  <title>DeisLabs Mixins</title>
  <updated>2019-03-31T12:29:29Z</updated>
  <link rel="self" type="application/atom+xml" href="https://porter.sh/mixins/atom.xml"/>
  <author>
    <name>DeisLabs</name>
    <uri>https://deislabs.io</uri>
  </author>
  <category term="exec"/>
  <category term="helm"/>
  <entry>
    <id>https://porter.sh/mixins/v0.4.0-ralpha.1+dubonnet/exec</id>
    <title>exec @ v0.4.0-ralpha.1+dubonnet</title>
    <updated>2019-03-31T12:29:29Z</updated>
    <category term="exec"/>
    <content>v0.4.0-ralpha.1+dubonnet</content>
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.4.0-ralpha.1+dubonnet/exec-darwin-amd64" />
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.4.0-ralpha.1+dubonnet/exec-windows-amd64.exe" />
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.4.0-ralpha.1+dubonnet/exec-linux-amd64" />
  </entry>
  <entry>
    <id>https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/exec</id>
    <title>exec @ v0.3.0-ralpha.1+caipirinha</title>
    <updated>2019-03-27T02:11:01Z</updated>
    <category term="exec"/>
    <content>v0.3.0-ralpha.1+caipirinha</content>
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/exec-darwin-amd64" />
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/exec-windows-amd64.exe" />
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/exec-linux-amd64" />
  </entry>
  <entry>
    <id>https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/helm</id>
    <title>helm @ v0.3.0-ralpha.1+caipirinha</title>
    <updated>2019-03-27T02:10:01Z</updated>
    <category term="helm"/>
    <content>v0.3.0-ralpha.1+caipirinha</content>
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/helm-darwin-amd64" />
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/helm-windows-amd64.exe" />
    <link rel="download" type="application/octect-stream" href="https://porter.sh/mixins/v0.3.0-ralpha.1+caipirinha/helm-linux-amd64" />
  </entry>
</feed>

Part of #150