fix js eval with '#!' strings, closes #10

This fixes Eval editor content function, when there's a '#!' in
JavaScript source, due to over-aggressive shebang filtering before
parsing. Now we only filter shebang strictly at the start of the file.

And rename clean-code to filter-shebang to make its purpose explicit.

(also fixes LightTable/LightTable#1744)
Merge branch 'pr/11'