HTML output should use absolute paths

[ericwb]

Bandit's HTML reports generate links to files. These links are relative
to Bandit's current working directory. If the output file is written
into a different directory, these links will be broken:

cd bandit/examples
bandit assert.py -f html -o ~/report.html
# The generated HTML links to "assert.py"

If the HTML reports are supposed to just be for filesystem access, then providing
absolute file:// URLs seems like a reasonable fix. If the HTML reports are intended
to live on a web server, then a different solution might be better.