Add support for footer docs

Enables a footer to be appended to the end of a generated document
sourced from tf files or documents in the same way as the header

Adds the `footer-from` field to the config yml
Adds the `--footer-from` flag to the cli

Signed-off-by: Simon Clifford <siclifford@gmail.com>
Signed-off-by: Khosrow Moossavi <khos2ow@gmail.com>

examples/.terraform-docs.yml

@@ -1,5 +1,7 @@
 formatter: markdown table
 header-from: doc.txt
+footer-from: footer.md
+
 sections:
   hide-all: true
   show:
@@ -7,6 +9,7 @@ sections:
     - inputs
     - providers
     - modules
+    - footer
 
 # output:
 #   file: README.md

examples/footer.md

@@ -0,0 +1,3 @@
+## This is an example of a footer
+
+It looks exactly like a header, but is placed at the end of the document