escape binary path only if it's executable

Fixes the issue came up in #66

Many people use special bin paths such as:
 - `export DISPLAY=:0;/bin/wkhtmltopdf`
 - `/usr/bin/xvfb-run -a -s "-screen 0 640x480x16" /opt/wkhtmltopdf/wkhtmltopdf --quiet`

These paths must not be escaped.
In this pull request, the escaped command is used only if the escaped string is executable.