fix word boundary w/ capture group

fixes BurntSushi/ripgrep#506. Word boundary search as arg had unexpected
behavior. added capture group to regex to encapsulate 'or' option search and
prevent expansion and partial boundary finds.

Signed-off-by: Evan.Mattiza <emattiza@gmail.com>

src/args.rs

@@ -505,7 +505,7 @@ impl<'a> ArgMatches<'a> {
     /// flag is set. Otherwise, the pattern is returned unchanged.
     fn word_pattern(&self, pat: String) -> String {
         if self.is_present("word-regexp") {
-            format!(r"\b{}\b", pat)
+            format!(r"\b(?:{})\b", pat)
         } else {
             pat
         }