Add custom css support to posterdown_betterport and posterdown_betterland (#103)

* added custom css to betterport

* added custom css to betterland
diff --git a/inst/rmarkdown/templates/posterdown_betterland/resources/template.html b/inst/rmarkdown/templates/posterdown_betterland/resources/template.html
index 01b31f9..f0d005f 100644
--- a/inst/rmarkdown/templates/posterdown_betterland/resources/template.html
+++ b/inst/rmarkdown/templates/posterdown_betterland/resources/template.html
@@ -298,6 +298,9 @@
   width: 3%;
 }
 </style>
+$if(custom_css)$
+<link rel="stylesheet" href=$custom_css$>
+$else$$endif$
 </head>
 <body>
 
diff --git a/inst/rmarkdown/templates/posterdown_betterport/resources/template.html b/inst/rmarkdown/templates/posterdown_betterport/resources/template.html
index b24e66e..e27de61 100644
--- a/inst/rmarkdown/templates/posterdown_betterport/resources/template.html
+++ b/inst/rmarkdown/templates/posterdown_betterport/resources/template.html
@@ -367,6 +367,9 @@
   color: #ffffff;
 }
 </style>
+$if(custom_css)$
+<link rel="stylesheet" href=$custom_css$>
+$else$$endif$
 </head>
 <body>