Update comparer.py - added support for schema selection

[rv-solomasov]

Adding the ability to select which schemas are being compared

[colonelpanic8]

@gianchub I'd like to use this feature. code seems good to me

[gianchub]

Hi @rv-solomasov and @colonelpanic8, so sorry I didn't get an email for this PR, but thanks to @colonelpanic8's mention I am now aware of it. The code seems good to me too, but can I ask @rv-solomasov to add tests to make sure the new feature works? I would add them myself but I have no time at the moment unfortunately. Thank you

[rv-solomasov]

@colonelpanic8 @gianchub Hi! I'm trying to think of an appropriate tests to add here but nothing really comes to mind :) Apologies as this is my first contribution of a kind and I'm a bit lost with the general practices. I'm using this particular code with some additional improvements for aligning schemas between mysql and postgres dbs and have not experienced any issues as the added changes are only applied to built-in arguments of various Inspector methods