new configuration for :thread command config[:thread_indent]

[no6v]

Current default (two spaces) is a bit long sometimes for me.

⚡ :eval config[:thread_indent] = ""
⚡ :thread $ab
|[$aa] ... vertical
|[$ab] ...
⚡ :eval config[:thread_indent] = nil # or
⚡ :eval config.delete(:thread_indent)
⚡ :thread $ad
|[$ac] ... reset to default
|  [$ad] ...
⚡ :eval config[:thread_indent] = Rational(1, 3)
⚡ :thread $af
|0/1[$ae] ... object responds `*' and accept single integer argument
|1/3[$af] ... this example doesn't make sense... it's your turn now :)

Enjoy!

PS: Should I write something in README or help text?

[jugyo]

thanks :)