Add a preview/dry-run command

[brycethornton]

One idea I had for pg-osc would be to do some quick observations about the table (and related tables) before actually starting the process. We could warn the user about any potential issues with large tables, constraints, etc. Or, we could just keep this functionality as it's own process that runs with a preview command instead of perform. Just thoughts. Here's what the output could look like:

pg-online-schema-change preview {all the same alter/connection options as perform}

Table to alter: table_name_from_alter_statement
Estimated number of records in table: 345,000,000
Size of table on disk: 30GB
Estimated time to write new table: 8 hours (based on a test run of 1% of your records)
Estimated time to validate constraints: 6 hours (based on ?)
Constraints to validate:
- Referential foreign key: column_name -> foreign_key_table (1.2B records) (WARNING: this could take a significant amount of time due to the large number of records)
- Referential foreign key: another_column_name -> another_table (200 records)
- Self foreign key: some_table has foreign key to table_name_from_alter_statement.id (20M records)
- Self foreign key: some_table2 has foreign key to table_name_from_alter_statement.id (2M records)
- Self foreign key: some_table3 has foreign key to table_name_from_alter_statement.id (2B records) (WARNING: this could take a significant amount of time due to the large number of records)

Suggestions:
- Given the large number of records in some related tables you may want to use the `--skip-foreign-key-validation` flag and then validate constraints separately to speed up the process.
- ???