src: improvements on #4: code review & add exmaple

- Code review pull request

- Add icon character on show_lines example to show that
even icon multibyte characters works

examples/show_lines.sh

@@ -16,6 +16,7 @@ while printf ''; do
 		{"text":"json with urgent flag", "urgent":true},
 		{"text":"json with highlight flag", "highlight": true},
         {"text":"multi-byte unicode: •"},
+        {"text":"icon unicode character: 😀"},
 		{"text":"json <i>with</i> <b>markup</b> <b><i>flag</i></b>", "markup": true},
 		{"text":"json <i>toggling</i> <b>markup</b> flag", "markup": $toggleMarkup},
 		{"text":"json <i>without</i> <b>markup</b> <b><i>flag</i></b>", "markup": false},

src/blocks.c

@@ -146,7 +146,7 @@ typedef struct
   utils
 ***************/
 
-void json_escape(char *in, char *out) {
+void json_escape(const char *in, char *out) {
     while (*in) {
         switch (*in) {
         case '\\':