Reading/writing of W3C-style embeded metadata in CSV, TSV files
Related to #2485 

The [W3C Tabular Data Model recommendation](https://www.w3.org/TR/tabular-data-model/) that include arbitrary text data, as well as column-specific metadata, such as column data types.

It would be very nice if Pandas could read metadata like this. There is [a section with an example of CSV/TSV meader metadata](https://www.w3.org/TR/tabular-data-model/#embedded-metadata) that might make a good starting point. The full recommendation seems somewhat vague, but perhaps that means that Pandas could help to define some more specific standards.

Perhaps a YAML header behind `#` characters, where some known variable names (e.g. `datatype`) are captured for use in reading the rest of the file, where remaining unused YAML data is added to a `df.metadata` dictionary?