Allow utf-8 safe meta key mapping in inputrc

[tompng]

Fixes #603

The inputrc file below will add "\ec" to additional key binding but does not add "\xE3". This behavior is different from Readline but I believe this is better.

# inputrc
"\M-c": "[You input M-c]"

Also supports these formats

"\M-\C-x": key
"\Meta-\C-x": key
"\Control-\Meta-x": key
Meta-c: key
Control-c: key
Meta-Control-c: key
C-M-c: key

Problem of Readline

Using this inputrc file,

# inputrc
"\M-c": "[You input M-c]"

"\M-c" is "\xE3". Readline only assign "\xE3" to the right hand side key. Terminal emulator that supports utf-8 does not send "\xE3" by alt+c because "\xE3" is part of a valid utf-8 string.
When you enter "あ" == "\xE3\x81\x82", Readline thinks the input is "\M-c" + [invalid two bytes].

ruby -rreadline -e "Readline.readline '>'"
>[You input M-c]??

If convert-meta is enabled, Readline assigns "\xE3" and "\ec". Now alt-c works.

# inputrc
set convert-meta on

But now, "え" == "\xE3\x81\x88" == "\xE3\x81\M-\C-H" will trigger \M-\C-H = ed-delete-prev-word.
In utf-8 environment, convert-meta is unusably terrible.

IMO Reline should not follow this old specification.
"\M-c": key should only assign "\ec" to the key.

[ima1zumi]

LGTM!