Print out executed code blocks when in debug mode

[mortenpi]

Makes Literate print out something like this

┌ Debug: execute_block(Main.##253, block)
│ ```
│ # Generation date and time
│ using Dates: now; now()
│ ```
└ @ Literate ~/Projects/highZ/dev/Literate/src/Literate.jl:708

for each code block it executes if e.g. JULIA_DEBUG=Literate.

Not sure if you want this, but I found it useful when debugging a case where Literate stalled while executing some code. This makes it a little bit easier to identify where it happens. If you want to merge this, I could also add a small note about this in the "Tips and tricks" section of the manual.

[fredrikekre]

I could also add a small note about this in the "Tips and tricks" section of the manual.

Yea that would be nice!

[mortenpi]

Done 🙂