Add a yaml DSL method.

I find the yaml format easier to read than csv (when working entirely in text, at least).  This loader works on this array-formatted yaml, but not (for whatever reason, possibly library brokenness) on multi-doc format:

```
- title: Foo
  rank: 2

- title: Bar
  rank: 3
```

lib/squib/api/data.rb

@@ -1,5 +1,6 @@
 require 'roo'
 require 'csv'
+require 'yaml'
 require_relative '../args/input_file'
 require_relative '../args/import'
 require_relative '../args/csv_opts'
@@ -67,6 +68,22 @@ module Squib
   end
   module_function :csv
   
+  # DSL method. See http://squib.readthedocs.io
+  def yaml(opts = {})
+    input = Args::InputFile.new(file: 'deck.yml').load!(opts)
+    import = Args::Import.new.load!(opts)
+    s = YAML.load_file(input.file[0])
+    data = Squib::DataFrame.new
+    # Get a universal list of keys to ensure everything is covered.
+    keys = s.map {|c| c.keys}.flatten.uniq
+    # Initialize the data frame; why is [] not the default value?
+    keys.each {|k| data[k] = [] }
+    # Load all cards into the frame, nil value if key isn't set.
+    s.each {|card| keys.each {|k| data[k] << card[k] } }
+    explode_quantities(data, import.explode)
+  end
+  module_function :yaml
+
   # Check if the given CSV table has duplicate columns, and throw a warning
   # @api private
   def check_duplicate_csv_headers(table)