Allow for passing -n option to whereami

[HeyNonster]

pry's implementation of whereami allows for passing quite a few arguments:

whereami --help
Usage: whereami [-qn] [LINES]

Describe the current location. If you use `binding.pry` inside a method then
whereami will print out the source for that method.

If a number is passed, then LINES lines before and after the current line will be
shown instead of the method itself.

The `-q` flag can be used to suppress error messages in the case that there's
no code to show. This is used by pry in the default before_session hook to show
you when you arrive at a `binding.pry`.

The `-n` flag can be used to hide line numbers so that code can be copy/pasted
effectively.

When pry was started on an Object and there is no associated method, whereami
will instead output a brief description of the current object.

    -q, --quiet                Don't display anything in case of an error
    -n, --no-line-numbers      Do not display line numbers
    -m, --method               Show the complete source for the current method.
    -c, --class                Show the complete source for the current class or module.
    -f, --file                 Show the complete source for the current file.
    -h, --help                 Show this message.

This commit is an attempt to work toward feature parity with pry by first implementing -n as an option to remove line numbers from the output.

In doing so, we've added a no_lineno kwarg to get_src and show_src, defaulting to CONFIG[:no_lineno] (which was previously hardcoded inside get_src).

This ensures that the default behavior remains the same but gives us the option of removing line numbers with an argument for whereami and possibly other commands in the future.

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[HeyNonster]

I think the main motivation for this comes from the documentation in pry:

The -n flag can be used to hide line numbers so that code can be copy/pasted
effectively.