Allow to redefine default MetaKernel magics with custom ones

[ccordoba12]

We're building an ipdb kernel for Spyder with MetaKernel. For that we defined Pdb commands as magics and some of their names collide with MetaKernel default magics.

With this PR users could create magics that are loaded instead of MetaKernel ones with the same name.

[coveralls]

Coverage decreased (-0.2%) to 75.028% when pulling c7b5b0e on ccordoba12:redefine-default-magics into 36fcede on Calysto:master.

[ccordoba12]

@blink1073, @dsblank, any chance of getting this one reviewed? We're needing it to keep working on our IPdb kernel.

Thanks!

[dsblank]

Sorry, just saw this! I'll take a look.

[ccordoba12]

Thanks!

By the way, this is a fantastic project!! Thanks a lot for working on it.

[dsblank]

@ccordoba12 This looks simple and fine; thanks! It would be great to document this someplace, maybe just in the doc string (I know there isn't a lot of docs). Or if not there, how about here:

https://github.com/Calysto/metakernel/blob/master/docs/source/installation.rst

[blink1073]

I also approve pending docs, thanks!

[ccordoba12]

I added a docstring to the Magic class, which is the one you look at to define your own magics, explaining how to redefine default magics in your own kernels.

Please let me know what you think about it.

[blink1073]

LGTM, thanks Carlos!

[blink1073]

@dsblank, are we ready to cut 0.21?

[dsblank]

@ccordoba12 Thanks! @blink1073 I think so. Thanks! I'll update the HISTORY with the other changes (and this one) soon.

[ccordoba12]

Thanks @blink1073 and @dsblank!