Add a .gitignore

Ignore some common temporary files.
Create digits-from

Create a function that will return all digits from a list.
Define digits-from in terms of filter
Use char-set-contains, from srfi/14
Create a letters-from function
Create a valid-country-prefix? function
Create a minimum-country-prefix-length function
Create allow-numeral-in-country-prefix?
Create letters+digits-from
Create random-from
Create generate-country
Create in-range?
Create bahamas-callsign?
Update README with generate-country

Update the README to reflect that we can now generate country prefixes
for callsigns.
Add generate-random-callsign

It generates random callsigns.
Move random-from to its own files

This is so we can use it from other places.
Add some functions that work with n-grams
Add Markov chain functions

We can now generate word-level similar texts based on a source text.
Add a pick function

It randomly picks from the supplied arguments based on embedded
probability counts.
Make the Markov chain more memory efficient