Add metadata

[curtisalexander]

Expand metadata captured:

• [x] file encoding
• [x] row count
  • may need error handling (check if < 0)
• [x] column count
  • may need error handling (check if < 0)
• [x] column names
• [x] column types
• [x] column labels
• [ ] column storage width
  • could query and use something like lazy static to create an appropriately sized Vec at runtime???
• [ ] column display width
• [x] file label
• [x] encoding
• [x] file format version
• [x] creation time
• [x] modified time
• [x] compression
• [x] endianness

[curtisalexander]

For a list of metadata to consider including, see the dump_metadata function from the readstat binary.

https://github.com/WizardMac/ReadStat/blob/master/src/bin/readstat.c#L392

[curtisalexander]

Expanded metadata as part of #38