Support for changing default kernel

[jdundondev]

It would be very useful to be able to change the default kernel. For example, by supporting
>>kernelname
to change to the specified kernel until otherwise specified.

cell 1 switches to sql kernel

>>sql
select * from t1

cell 2 continues with sql kernel

select * from t2

cell 3 python

>python
print("Hello Jupyter");

cell 4 back to sql

select * from t3

cell 5 back to python until changed

>>python
print("Hello Europa");

cell 6 continues with python

print("Hello Io");