fread yaml could support column attributes

[jangorecki]

csv yaml is a nice feature but most of its functionality can be handled by fread arguments like colClasses. What cannot be easily handled by fread args are attributes of data. Examples where it could be useful:

• we can store a factor column as integer, and in yaml metadata provide factor class as well as a list of factor levels (that obviously won't be feasible for many levels)
• we can store POSIXct field as numeric and in yaml metadata provide POSIXt class as well as origin and tz attributes.

Related to #1656

[MichaelChirico]

yaml= argument is in maintenance mode