Add ability to specify alternate path to i18n.toml #155

I'm working on a fairly large project right now that uses i18n-embed, among other tools, and I prefer not to fill up the root directory with individual configuration files. It would be nice to have the option to specify a path to an alternate location for i18n.toml via a field in the Cargo manifest, like so:

[package]
name = "example"
# ... more standard cargo options

[package.metadata]
i18n-config-path = "conf/i18n.toml"

I would be happy to contribute the implementation if this is a change you're willing to accept. Thanks for your work on this project!