Allow multiple preloads before entering REPL

[finiteprods]

extends the preload command line option to accept multiple files

• resolves Multiple preloaded files #340
• preloads files in sequence, e.g.

$ lurk repl --preload demo/simple.lurk demo/mini-mastermind.lurk
Loading demo/simple.lurk
[1 iteration] => 123
[2 iterations] => 2
square
[5 iterations] => 64
make-adder
five-plus
[6 iterations] => 8
Loading demo/mini-mastermind.lurk
length
maybe-remove
score
code-valid?
score-one-turn
t
t
t
t
Lurk REPL welcomes you.
lurk-user>