Update to v0.1.2 with HTML support

moved posterdown_pdf to posterdown_latex, added posterdown_html, added to ReadME, added ciation file. added @shedimus  as contributor in Description file.
diff --git a/inst/rmarkdown/templates/posterdown_pdf/template.yaml b/inst/rmarkdown/templates/posterdown_pdf/template.yaml
index 6b13ddb..3fba43a 100644
--- a/inst/rmarkdown/templates/posterdown_pdf/template.yaml
+++ b/inst/rmarkdown/templates/posterdown_pdf/template.yaml
@@ -1,4 +1,4 @@
-name: Posterdown PDF
+name: Posterdown PDF (Legacy)
 description: >
- Template for creating R Markdown based PDF via Latex
+ Template for creating R Markdown based PDF via LaTeX. Legacy Option, no updates past Feb 6 2019.
 create_dir: true