Add --single-transaction

[tvb]

Missing --single-transaction config option:
https://dev.mysql.com/doc/refman/5.1/en/mysqldump.html#option_mysqldump_single-transaction

[gocom]

Danpu currently uses locking, that is mutually exclusive with transactions. It locks all tables before it starts taking a backup, which too leads to consistent state of data.

Will see about adding this for the feature release, 2.7.0.