Skip symbol lookup for CSV import

[gjmveloso]

Apparently the CSV Import format is currently limited to securities that are available on Yahoo Finance.

It would be great to have a column that signals the import process to skip symbol lookup, in the same way the manual add interface have it (under Other).

In the following example, APV is not a publicly trade-able symbol rather a code for my broker app related to one of their mutual funds.

May also be a way to work around #62