add option to open specific files in the browser

[RonnyPfannschmidt]

context, i'm using live-reload to auto-update views on reveal.js presentations in markdown files

so i have a directory tree like

repo:
-  assets: 
      | git submodule pointing to reveal.js
-  mytalk:
   - mytalk.html
   - mytalk.md
- my-othertalk: 
    ...
- tox.ini

where tox.ini just spells something like

[tox]
skipsdist=true
envlist=live
[testenv:live]
skip_install=true
deps =
   livereload
commands =
 	livereload {{posargs}}

i would like to be able to invoke something like
tox -- --open-in-browser mytalk/mytalk.html

to have live-reload open $hostname:$randomport/mytalk/mytalk.html
instead of going to the server and adding the path there