New Feature: ExternalData DataSource support multiple files

[breed052]

In some cases where ExternalData's DataSource tag is being used, multiple combinations of smaller files would be more helpful than having one monolithic csv file. This would allow tailoring each scenario outline to a needed combination instead of having a specific file for each scenario. The files to be used could be separated by comma, or multiple tags of the same DataSource could be combined.

Example:

@DataSource:setA.csv,setB.csv
Scenario Outline: Test which requires both sets
 ...

@DataSource:setA.csv
Scenario Outline: Test which requires only one set

[gasparnagy]

If we would do this, I would rather vote for allowing multiple @DataSource tag instead of including a comma:

@DataSource:setA.csv
@DataSource:setB.csv
Scenario Outline: Test which requires both sets
 ...

@DataSource:setA.csv
Scenario Outline: Test which requires only one set

Comma is also valid in file names, so it would be confusing. Also I feel that tooling can better handle a one-tag-one-reference approach.