add middle_fontsize option to YAML

adds option to change font size of the middle text in the poster using the YAML header. See issue #56
diff --git a/inst/rmarkdown/templates/posterdown_betterland/resources/template.html b/inst/rmarkdown/templates/posterdown_betterland/resources/template.html
index c71911e..65bcf99 100644
--- a/inst/rmarkdown/templates/posterdown_betterland/resources/template.html
+++ b/inst/rmarkdown/templates/posterdown_betterland/resources/template.html
@@ -238,7 +238,7 @@
 padding-left: 5%;
 padding-right: 5%;
 padding-top: 20%;
-font-size: 170px;
+font-size: $middle_fontsize$;
 text-align: left;
 }
 .fab {
diff --git a/inst/rmarkdown/templates/posterdown_betterland/skeleton/skeleton.Rmd b/inst/rmarkdown/templates/posterdown_betterland/skeleton/skeleton.Rmd
index d3f1e95..51d39fc 100644
--- a/inst/rmarkdown/templates/posterdown_betterland/skeleton/skeleton.Rmd
+++ b/inst/rmarkdown/templates/posterdown_betterland/skeleton/skeleton.Rmd
@@ -28,6 +28,7 @@
 #Middle of the poster
 middle_fontfamily: "Special Elite"
 middle_textcol: "#FFFFFF90"
+middle_fontsize: "17px"
 main_findings:
   - "Make **better posters** with <br> RMarkdown + **posterdown**."
   - "Transition from **poster** to **manuscript** with ease!"