Refator Hash#fetch test

LinkerConfig is a struct with five params

chop with utf-8. fix #2967

support Regexp literal option: //n and //u

fix tests on windows.

'bin/mruby' not work on windows. so correct command name and quoted arguments.

Small Markdown updates to README.md

Small copy and styling updates

fix signature of mrb_class_new_instance()

fix String#inspect with MRB_UTF8_STRING

dd925578 changed the order of arguments,
but it doesn't seem intentional.

cf. #2963

define MRB_UTF8_STRING (in mrbconf.h) to enable UTF-8 support.

C API documentation

some routines need to refer mruby objects (e.g. callbacks), in that case
you have to protect your objects from garbage collection. the new functions
mrb_gc_register() keeps those objects from GC. you have to remove your
objects using mrb_gc_unregister() when your C routines use mruby objects
any longer, otherwise objects will leak.