Add option to output DML lines about nebula-importer

[veezhang]

@wey-gu Maybe studio should use csv as datasets instand of ngql?
csv has the following advantages:

• smaller size
• better readability
• only care about data, nothing to do with syntax

[veezhang]

@wey-gu You just want to store the statement to a file instead of storing in nebula.?

[wey-gu]

@wey-gu You just want to store the statement to a file instead of storing in nebula.?

Yes, I generated a small dataset as CSV and was preparing for studio starter dataset, so I did this with above change :-P

Not sure how general this requirement could be to other users.

[veezhang]

@wey-gu So, why not use nebula-importer tool to import directly?

1. generated csv to ngql
2. use studio to exec ngql

=>

1. use nebula-importer import csv

[wey-gu]

@wey-gu So, why not use nebula-importer tool to import directly?

1. generated csv to ngql
2. use studio to exec ngql

=>

1. use nebula-importer import csv

Yes, for pure import purposes, that's enough, only just this use case I had to prepare files for this

Maybe this isn't a general requirement at all.