Add --lines option

Hi,

Thanks very nice command.
I felt better if we can see only several recent lines of changelogs instead of the whole in pager,
so, I added `--lines`, or `-n`, option to `gem changelog` command.

You may use it like below after merging this patch.

    $ gem changelog -n 6 gem-changelog
    ## 1.0.2 (2013-03-05)
    
    - Add "History" to the changelog patterns.
    - Update CHANGELOG.md
    
    ## 1.0.1 (2013-03-05)

Or, you may also specify the number of lines by writing `.gemrc` like this:

    gem: --lines=20

I'm so glad if you will merge this patch.
Could you consider?

Thanks.