Allow using cascade when truncating tables

[jaitaiwan]

In cases where a data in a database has foreign key references against
the table we're wanting to truncate, we can use the CASCADE key to
ensure that all data referencing the rows we're deleting are also
deleted to ensure the database keeps it's integrity.

• Added in-code documentation as per crystal spec for the truncate
  command
• Added new cascade flag to truncate method, set to "false" by default
  (potentially dangerous)
• Added migration ensuring foreign key constraint on posts by comments
• Added specification to test cascade deletes data referenced by
  foreign keys

Fixes #648

Some possible considerations:

• Perhaps we should turn on cascade by default considering it's potentially a dangerous state to truncate ignoring foreign key constraints? However I know that cascade by default would delete even more data so perhaps that's unwarranted
• Technically speaking the spec I added is really only testing if Postgres is doing a cascading delete, I'd feel much more comfortable if the resulting ExecResult was inspectable to see what the final query that was run was so that I could do an assertion against it.

[jwoertink]

Sweet! Great job on this. I think you just need to run the formatter crystal tool format spec src, and make sure ameba is happy ./bin/ameba. I made a few suggestion, but once those are done, I think this will be good!

[jwoertink]

Sweet! Thanks for the contribution.