Execute Cell and Move should be default

[cgahr]

In general, when you use jupyter notebooks, you want to execute a cell and jump the next one, just like the default behaviour for "normal" notebooks.

The same should be true for Helium. As such Helium: Execute Cell and Move should be the default and not Helium: Execute Cell. This small change (less than 10 letters) makes a very big difference and dramatically increases the handling of Helium.

Any opinions?

[cgahr]

commit e75998c implements this behavior. Should this be the new standard?

[cgahr]

Closed with #140