Support editing command input in editor

[st0012]

Flow:

• User types the target command (note: edit command has a different semantic as of now) in IRB
• IRB opens a temporary file in thed configured editor (same as the edit command uses)
• After the file is saved & closed, IRB executes its content

[elfham]

In vi-mode, such functionality is supported via the v (vi_histedit) command.