Additional quoting options for field data types

In order to support CSV processors which distinguish between strings and
non-string values, add the quotedString option which unconditionally
quotes all string values.  This option can be used with the quoted
option to quote empty string values, but it does not override the quoted
option (as I can not currently think of a use case where quoting
everything but strings would be desirable).

In order to support CSV processors which distinguish between
missing/unspecified values and empty strings (i.e. null vs empty
string), add the quotedEmpty option to specify whether empty values are
quoted.  This option overrides quoted and quotedString to allow users to
allow quoting only empty values (to avoid the overhead of quoting all
strings while still distinguishing empty from missing values) and to
avoid quoting empty strings when they should be treated as missing by
the CSV consumer.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Merge pull request #7 from kevinoid/quote-data-types

Support additional quoting options for fields based on data type