Add mruby-bin-mruby-config

mruby command should not execute program on "syntax only option(-c)".

fix indent.

fix off-by-one error in attrsym

The allocation size for null-terminated character is lacking.
In actual, it is no problem in most case
because codegen_palloc allocates more a memory size than required size.

Add notes about new features.

Fix #1793

fix off-by-one error in String#rindex(fixnum)

fix a comment end of include guard

null-terminated string should not be included in search targets.

add Integer#div and Float#div to mruby-numeric-ext

fix Float#divmod returns nil

Introduce mrb_exc_new_str_lit for C string literals

`bintest` improvement.

fix include guard style.

fix a memory leak in mruby-random.