Handle platform-specific sed -i requirements.

`sed -i` on macOS requires an extension, but on Linux that extension can
cause errors. Use a platform check to only include the extension on
macOS.

resolves gh-5

hosts

@@ -56,6 +56,20 @@ export _SPACE_=$' '
 export _TAB_SPACE_="${_TAB_}${_SPACE_}"
 export _TAB_SPACE_CC_="[${_TAB_SPACE_}]"
 
+# $SED_I_ARG
+#
+# `sed -i` takes an extension on macOS, but that extension can cause errors in
+# GNU `sed`.
+#
+# https://stackoverflow.com/q/43171648
+# http://stackoverflow.com/a/16746032
+if sed --help >/dev/null 2>&1
+then # GNU
+  export SED_I_ARG="-i"
+else # macOS
+  export SED_I_ARG="-i ''"
+fi
+
 ###############################################################################
 # Debug
 ###############################################################################
@@ -811,9 +825,7 @@ disable() {
 
     printf "Disabling:\\n%s\\n" "${_targets}"
 
-    # -i '' - in place edit. BSD sed requires extension argument, for GNU
-    # it's optional. More info: http://stackoverflow.com/a/16746032
-    sed -i '' \
+    sed "${SED_I_ARG}" \
       -e "s/${_regex_ip}/\\#disabled: \\1/g" \
       -e "s/${_regex_commented_hostname}/\\#disabled: \\1/g" \
       -e "s/${_regex_hostname}/\\#disabled: \\1/g" \
@@ -907,9 +919,7 @@ enable() {
 
     printf "Enabling:\\n%s\\n" "${_targets}"
 
-    # -i '' - in place edit. BSD sed requires extension argument, for GNU
-    # it's optional. More info: http://stackoverflow.com/a/16746032
-    sed -i '' \
+    sed "${SED_I_ARG}" \
       -e "s/${_regex_ip}/\\1/g" \
       -e "s/${_regex_commented_hostname}/\\1/g" \
       -e "s/${_regex_hostname}/\\1/g" \
@@ -1113,18 +1123,14 @@ remove() {
     done
   fi
 
-  # Regular Expression Notes
-  #
-  # -i '' - in place edit. BSD sed requires extension argument, for GNU
-  # it's optional. More info: http://stackoverflow.com/a/16746032
   if ((_is_search_pair))
   then
-    sed -i '' \
+    sed "${SED_I_ARG}" \
       -e "/${_regex_ip_hostname_commented}/d" \
       -e "/${_regex_ip_hostname}/d" \
       "${HOSTS_PATH}"
   else
-    sed -i '' \
+    sed "${SED_I_ARG}" \
       -e "/${_regex_ip}/d" \
       -e "/${_regex_commented_hostname}/d" \
       -e "/${_regex_hostname}/d" \