echo: Implement -e flag

This lets you do things like `echo -e '\e[31mhello\e[m'` to print
red text, without having to resort to the tyranny of printf(1).

Userland/Utilities/echo.cpp

@@ -9,6 +9,58 @@
 #include <stdio.h>
 #include <unistd.h>
 
+static char backslash_escaped_char(char c)
+{
+    switch (c) {
+    case '\\':
+        return c;
+    // `\"` produces `"` with printf(1), but `\"` with echo(1)
+    case 'a':
+        return '\a';
+    case 'b':
+        return '\b';
+    case 'e':
+        return '\e';
+    case 'f':
+        return '\f';
+    case 'n':
+        return '\n';
+    case 'r':
+        return '\r';
+    case 't':
+        return '\t';
+    case 'v':
+        return '\v';
+    default:
+        return c;
+    }
+}
+
+static String interpret_backslash_escapes(String s)
+{
+    StringBuilder builder;
+
+    for (size_t i = 0; i < s.length();) {
+        if (char c = s[i++]; c != '\\') {
+            builder.append(c);
+            continue;
+        }
+        if (i == s.length()) {
+            // Last character of string is '\' -- output it verbatim.
+            builder.append('\\');
+        }
+
+        char c = s[i++];
+        if (c == 'c') // `\c` suppresses further output.
+            break;
+        // FIXME: \0ooo, \xHH, \uHHHH, \UHHHHHHHH should produce characters if followed by
+        // enough digits.
+        builder.append(backslash_escaped_char(c));
+    }
+
+    return builder.build();
+}
+
 int main(int argc, char** argv)
 {
     if (pledge("stdio", nullptr) < 0) {
@@ -18,13 +70,18 @@ int main(int argc, char** argv)
 
     Vector<const char*> values;
     bool no_trailing_newline = false;
+    bool should_interpret_backslash_escapes = false;
 
     Core::ArgsParser args_parser;
     args_parser.add_option(no_trailing_newline, "Do not output a trailing newline", nullptr, 'n');
+    args_parser.add_option(should_interpret_backslash_escapes, "Interpret backslash escapes", nullptr, 'e');
     args_parser.add_positional_argument(values, "Values to print out", "string", Core::ArgsParser::Required::No);
     args_parser.parse(argc, argv);
 
-    out("{}", String::join(' ', values));
+    String output = String::join(' ', values);
+    if (should_interpret_backslash_escapes)
+        output = interpret_backslash_escapes(move(output));
+    out("{}", output);
     if (!no_trailing_newline)
         outln();
     return 0;