YAML support

[chungers]

This PR

• adds support for YAML as input through all CLI operations.
• the CLI operations have been standardized to support templates.
• templates can be in YAML or in JSON. The default is JSON. Use -y to use YAML.
• Where positional args are taken as filename in the CLI (e.g. infrakit group commit <file> has been changed to support a template or stdin, if the positional arg is -.
• Support for INI file decoding, used for decoding AWS credential files.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "yaml" [email protected]:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353866832
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #457 into master will decrease coverage by 0.18%.
The diff coverage is 23.68%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
- Coverage   68.13%   67.94%   -0.19%     
==========================================
  Files          62       68       +6     
  Lines        3574     4065     +491     
==========================================
+ Hits         2435     2762     +327     
- Misses        934     1063     +129     
- Partials      205      240      +35

Impacted Files	Coverage Δ
pkg/template/template.go	71.42% <ø> (ø)
pkg/template/funcs.go	67.77% <23.68%> (ø)
pkg/template/fetch.go	36.84% <0%> (ø)
pkg/template/defaults.go	71.42% <0%> (ø)
pkg/template/util.go	63.63% <0%> (ø)
pkg/template/help.go	67.34% <0%> (ø)

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