Support sqlite database

[lpantano]

There should be a option like mirtop convert --format sql to convert a mirgff3 file into a table inside a sqlite database.

We can start with creating a script that takes a mirgff3 file and converts to sqlite file. Ideally we should keep all the information, I think the output of mirtop counts --add-extra would be a good table to directly add in a sqlite database file.

After that:

• integrate the script in the mirtop code
• add test units
• start with filter or/and query function from the database