Support all magics available in xeus-cling

[anutosh491]

I currently see xeus-cling suporting few magics (https://xeus-cling.readthedocs.io/en/latest/magics.html)

• %%file
• %%timeit
• %%executable

I think %%file through os.hpp/os.cpp should work right off the bat, so that can be enabled here

xeus-cpp/src/xinterpreter.cpp

Line 407 in 381f05d

// preamble_manager["magics"].get_cast<xmagics_manager>().register_magic("file", writefile());

These should be supported through a plugin system eventually I think (#140)

[anutosh491]

Just framed some dummy code for %%timeit (untested ... somebody could pick it up and test it)

https://gist.github.com/anutosh491/8ef8fddac545acfad05039164679ecf4