Export `rl_readline_name` cell to Ruby interface

[RoUS]

IIUIC, the rl_readline_name attribute allows per-application customisations in .inputrc. From man readline concerning the $it application/$endif construct:

---

application
The application construct is used to include application-specific settings.
Each program using the readline library sets the application name,
and an initialization file can test for a particular value. This could be used
to bind key sequences to functions useful for a specific program. For instance,
the following command adds a key sequence that quotes the current or
previous word in bash:

  $if Bash
  # Quote the current or previous word
  "\C-xq": "\eb\"\ef\""
  $endif

---

The current implementation arrogates this capability to itself, preventing its use by Ruby application developers. Currently the readline gem hard-codes this to (char *)"Ruby". However, a compiler (interpreter) isn't really an application, particularly since it itself is not using the readline library. That seems to be more appropriately the province of things written in Ruby. And then only if they choose to change it from the "Ruby" default. Note that the documentation explicitly says: "Each program using the readline library sets the application name." I offer that the Ruby interpreter binary is not "the application" by this documentation.

Therefore, this is a FREQ that this cell be exposed to the Ruby level in the gem code, allowing Ruby code developers to set this according to their own application needs.

Thanks!