Add echo

src/echo.c

@@ -0,0 +1,14 @@
+#include <string.h>
+#include <stdio.h>
+#include <unistd.h>
+
+int
+main(int argc, char *argv[])
+{
+  while(*argv) {
+    (void)fputs(*argv, stdout);   // Print argv
+    if(*++argv) putchar(' ');     // If multiple things in argv, print a space between them.
+  }
+  putchar('\n');
+  return 0;
+}