Allow reading of line numbers of tables, arrays and keys.

[oshawa-connection]

Apologies as this is quite rough and needs some review, but this is a PR for issue #74.

I hacked together two files just for demo purposes, they can be safely deleted: toml_line_number_example.c and line_number_sample.toml

The one I did struggle with is getting the line_no directly on toml_datum_t as it is created from a toml_raw_t which I don't want to change into a struct from const char *. Instead you have to use toml_key_lineno for this.

[w568w]

Sorry for spamming, but what is keeping this PR from being reviewed for 2 years? This does not look like a very complicated pull request and there is no breaking change made.