Feature: Adds merge command

Adds merge command for merging multiple yaml files together.

Resolves: #31

examples/data1.yaml

@@ -0,0 +1,2 @@
+a: simple
+b: [1, 2]

examples/data2.yaml

@@ -0,0 +1,3 @@
+a: other
+c:
+  test: 1

examples/data3.yaml

@@ -0,0 +1,5 @@
+b: [2, 3, 4]
+c:
+  test: 2
+  other: true
+d: false