Bugfix: make "safe" char offset actually safe

The debugger crashed when stopping at a statement that included a blank line. Golfed test case:

{1

} 

I did not see any new problems when testing this bugfix with the test case, some variants of it, and with a 200 line script that I'm working on and which happened to trigger this bug.
Speedup display of regex match

Only subst in the couple hundred chars around the current match position.

If the target is hundreds or thousands of lines long then this is always going to be a lot faster. Right now the impact is colossal for target strings of more than a dozen typical text file lines.
Merge pull request #20 from raiph/patch-1

Bugfix: make "safe" char offset actually safe
Merge pull request #22 from raiph/patch-5

Speedup display of regex match
Merge pull request #1 from jnthn/master

10x+ speedup when 100 line script with breakpoints