support coalesce struct tag

it marshals NULL values to the zero type

allow specifying table names

`foo.bar` can be specified as `db:"bar"` or `db:"foo.bar"`.
Embedded structs can be set to `db:"foo.*"`, and their children
are treated as belonging to table foo.

allow specifying table names

`foo.bar` can be specified as `db:"bar"` or `db:"foo.bar"`.
Embedded structs can be set to `db:"foo.*"`, and their children
are treated as belonging to table foo.

fixes an unintended change in the way sqlx.In deals with []int and []…

…string inputs

version 1.3.0

add go.sum and updated go.mod w/ test deps

Merge pull request jmoiron#118 from jmoiron/liaoliaopro-master

tag handling for reflectx

update README to refer to shiny new docs