rfc wishlist for a repl

[monkyyy]

With file watching I made a small repl fairly quickly and its a 
tool that I kept using for longer then a week suggesting its 
worth keeping around

Its currently a bit of a piece of shit but its competing with 
dlang.io in my use as is: 
https://github.com/opendlang/opend/issues/96

(btw dlang.io is consistently degrading someone should do a clean 
up pass, fix the resharing, shorten the all compiler list, add 
opend)

rough ideas:
* decrease io by detecting if files change(its a little slow)
* track diffs
* align reported error message possibly using whatever line 
pragma or just padding if it fails
* pull out har file with a flag
* a/b test different commands