Windows: Handle lack of quotes in browser command line

[amodm]

It seems like in some cases, the browser command line as determined by AssocQueryStringW may not be appropriately quoted, e.g. the case here.

This PR modifies the behaviour on Windows to handle this situation, as follows:

1. If the command line has quotes, process normally
2. If the command line does not have quotes, keep trying spaced tokens until it feels like we've come across a valid exe (as determined by path existence), quote it, and process the revised command line.

Fixes #68