Add 'format_options' option

I'd like to use ALFC's char_handlers and block_handlers options with this middleware.
Currently there are no path to pass these options to underlying ALFC,
so I added 'format_options' to AxsLog's options.

One alternative interface might be:

enable 'AxsLog',
    format => ...
    char_handlers => ...
    blchar_handlers => ...

I am not sure which is the better.
Merge pull request #1 from zentooo/format_options

Add 'format_options' option