Fix #8 use shiny::isTruthy on replacement in addin

DESCRIPTION

 Package: regexplain
 Title: Rstudio Addin to Explain, Test and Build Regular Expressions
-Version: 0.2.0.9000
+Version: 0.2.1
 Date: 2018-04-04
 Authors@R: c(
     person("Garrick", "Aden-Buie", email = "g.adenbuie@gmail.com", role = c("aut", "cre")),

NEWS.md

 ## regexplain 0.2.x
 
+### 0.2.1
+
+* Fix addin crash when replacement function is visited but replacement field
+  is not initated (#8).
+
 ### 0.2.0
 
 * Added "Library" button that opens a dialog to import regex patterns sourced 

R/regex_gadget.R

           options <- paste0(options, "=TRUE", collapse = ", ")
           pattern <- paste(pattern, "#", options)
         }
-        if ("regexFn_replacement" %in% names(input) && replacement() != "") {
+        if ("regexFn_replacement" %in% names(input) && isTruthy(replacement())) {
           pattern <- paste0(
             pattern, "\n",
             'replacement <- "', escape_backslash(replacement()), '"'