Safely decode non-ascii SQL queries for display

SQL queries containing non-ascii byte strings would cause errors, both with and
without Pygments highlighting.

This updates the non-Pygments case to handle a simple decoding to ensure the
value is ascii-safe. It also removes passing an explicit "utf-8" encoding to
Pygments, since this causes errors when the bytes are not utf-8. When the
encoding is omitted, Pygments will default to "guess" the encoding by trying
utf-8 and falling back to latin-1.

Fixes #55