Please support working without xdg-utils on Linux

[joshtriplett]

On Linux, opening the default browser uses xdg-open, part of xdg-utils; however, nothing in a default desktop install requires the installation of xdg-utils. Please consider supporting some fallbacks, such as gvfs-open, or parsing the $BROWSER environment variable directly.

[amodm]

Thanks for the input @joshtriplett. I'd originally sourced the choice of command on Linux from this stackoverflow discussion.

Will address this as:

1. Use browser variable if available
2. Use xdg-open as a fallback

I think using the browser variable should be sufficient instead of adding something like gvfs-open

[joshtriplett]

@amodm Thanks for the update. I mostly mentioned gvfs-open because unlike xdg-open, it's installed by default in GNOME environments, and it still respects users' app selections in the GNOME preferences (which don't set $BROWSER). xdg-open is definitely the right tool to start with, but it'd be nice to have fallback commands that are more likely to be installed.

[amodm]

@joshtriplett incorporated in v0.2.2 on crates.io