Add prompt support

[davmacario]

Tested - successful. Closes #12

Prompt can be provided either:

• As a string (--prompt "Here is the recipe for pizza: "); the same prompt will be used for all samples
• As a file - each paragraph will be a separate prompt for a different sample. Notice that the \n is kept.

Also allows to generate of fewer samples than n. of nodes, and raise a warning in this case.