Add specifications

[peterdesmet]

The specifications are currently copied from the data-validator repository to this one in the write-specifications branch: https://github.com/inbo/whip/blob/write-specifications/docs/specifications.md

These need to be reviewed, edited and accepted to the master branch.

[peterdesmet]

Progress of my review:

• [x] A single specification
• [x] Multiple specifications
• [x] A specification file
• [x] allowed
• [x] minlength
• [x] maxlength
• [x] stringformat
• [x] regex
• [x] min
• [x] max
• [x] numberformat
• [x] mindate
• [x] maxdate
• [x] dateformat
• [x] empty
• [x] delimitedvalues
• [x] if

[peterdesmet]

Finished in https://github.com/inbo/whip/commit/5468ffc2b4b0ab163eca4e1f5acb2470cdb95a26