Move htmlwidgets to suggests (closes qinwf#18)

DESCRIPTION

@@ -22,13 +22,20 @@ License: BSD_3_clause + file LICENSE
 LazyData: TRUE
 Depends: R (>= 3.3)
 Imports:
-    Rcpp (>= 0.12.2), stringi, RcppParallel,
-    htmlwidgets (>= 0.6),
-LinkingTo: Rcpp, RcppParallel
+    Rcpp (>= 0.12.2),
+    stringi,
+    RcppParallel
+LinkingTo:
+    Rcpp,
+    RcppParallel
 Suggests:
-    knitr (>= 1.12.3),testthat,
-    rmarkdown (>= 0.9.5),microbenchmark,
-    ggplot2, directlabels
+    htmlwidgets (>= 0.6),
+    knitr (>= 1.12.3),
+    testthat,
+    rmarkdown (>= 0.9.5),
+    microbenchmark,
+    ggplot2,
+    directlabels
 URL: https://github.com/qinwf/re2r/
 BugReports: https://github.com/qinwf/re2r/issues
 VignetteBuilder: knitr

R/widget.R

@@ -34,11 +34,15 @@ remove_named = function(string){
 #' @export
 show_regex <- function(pattern, width = NULL, height = NULL) {
 
-    if (is.character(pattern)){
+    if (!requireNamespace("htmlwidgets", quietly = TRUE)) {
+        stop("'htmlwidgets' package is not installed.")
+    }
+
+    if (is.character(pattern)) {
         res = re2(pattern)
-    }else if (inherits(pattern,"re2c")) {
+    } else if (inherits(pattern,"re2c")) {
         res = pattern
-    } else if (is.list(pattern)){
+    } else if (is.list(pattern)) {
         return(show_regex(pattern[[1]], width, height))
     } else {
         stop("pattern is a string or a pre-compiled regular expression.")

R/zzz.R

@@ -32,7 +32,6 @@
 
 ##' @useDynLib re2r
 ##' @import Rcpp
-##' @import htmlwidgets
 ##' @importFrom stringi stri_enc_toutf8
 ##' @importFrom RcppParallel RcppParallelLibs
 NULL