Get line number of toml table or value in file

[oshawa-connection]

First off, thank you so much for making this library :)

Is there a way to get the line number of a table within a file? If not, is it a good idea to implement? I'd be happy to do the pull request if it is!

For example, if a config file has an invalid value (like there is a string value that must be one of a set number of values), someone could use this function to print out "invalid value at line X" to help find and fix their config file for their app.

[oshawa-connection]

Created a PR for this #75