Enh(importer): add headerFilter and objectCallbacks

`gridOptions.importerHeaderFilter` allows you to match the i18n version
of a header in a csv file.
`gridOptions.importerObjectCallback` allows you to massage the created
object, for example to convert decodes into coded values.
Merge pull request #1835 from PaulL1/importer_extensions

Enh(importer): add headerFilter and objectCallbacks