run.ceylon: Verify input file argument

and don’t fall back to '../ceylon-walkthrough/source/en/01basics.ceylon'
Add a countingWriter to FormattingWriter

The countingWriter sits between the FormattingWriter and its underlying
Writer and keeps track of the length of the currently written line.

Needed to implement #12.
Correctly implement tab width

the width of a tab isn’t static – a tab aligns the line to the next
multiple of tabWidth.