Fallback to non-colored inspect when coloring takes too long

[osyoyu]

Inspecting large objects may take a long time since IRB colors the entire output of #inspect all at once. This patch lets IRB to give up coloring after a certain time and fall back to non-colored output. Changes can be observed by inspecting large Arrays, for example [1] * 1000000.

The default timeout is 2 seconds, but this can be configured through IRB.conf[:INSPECT_COLORING_TIMEOUT]. Passing nil to this option will disable timeouts (IRB will never fall back to non-colored output).

[st0012]

Thanks for the PR 👍 However, it will not be the best solution to the problem because using timeout will cause hanging problems with debug for the irb:rdbg integration. See https://github.com/ruby/debug/issues/877

Maybe we should improve colour printer instead?