feat: display nullability of columns

[micalevisk]

closes #51

A column is said NOT NULL if nullable option of @Column() is undefined or false

The example examples/shopping-cart looks like this:

screenshot

http://www.plantuml.com/plantuml/png/xLd_J-Cs5FyzVuNNILChWQ1Cx7IdNdKzA7VLW9jORjgaIIStVcsj9d5cEvGExd-_PoTDapH92mSQI2iIr7uVlsU_xpRlbAPInu7lE3ymcF0GYAPZ7zeGqW3s21kRlnq2eUPwILgzSTzIUWVZ_jzXRo92JHJ_5zvs3uadV27VJYLTzjlfkjw1GVHRQ-xH79RjcmwvdN3msLU237jsuPjJAkSVXCyWl1Ait_i9WVtUWQNroOqHc36Pb9aa77uSsC_6scmTJiH1MBLoU6P-S6K0ZDDz2PwGh8g5hESnwBu-Uzt4mvbn1aHfa23qCWALJ0IWPuAb0uzBpmV7qI8YMf2nq5e4X77afha87KVDUHXHIGCYHHmob1629CUvUH_tQTWJ8P5nZg1cb8a53wTaGdq5ESgCXcmXkTOGfhGSqVEfKkJM8VXvJxtvr0e-CQSYYndNO2a3ASKYcLH0fn8WJ5QW4a2M25yTFA7rCtA2_ZVI4EmH3xzzGNO1wH2gSaCZsleDuOnqtf3tem-ZIxTzTDWXIB7EG28fMEpfKHdGwvtzskzdCCEq7hCoBb7ps63-65oTV1nSjRk7YQYC7adkWK6SdNyQkEtkKP7CkF9GHxeUyKzCauX2nEyNW_DpivTVYXYA5h-sKjpHvLy6q2q2F0bK0njOEQS3T-YEBeO6iGB44Mi6U1YTaEpJ7Vxf4cEOp2uQfa8kZLsIRokpfAETh48bijz1uQWB7_8gVtNSoMyKOuIwPSo6MPGVJwi0EnptjyH_Bb3EtigPlXYfoQaBCu_YZzcc4PxVy_5zjtm5rnmMwqrBEovkEvrxn41wk6Kth2QfDZcf8HAYUEnpDGFsFS6GCNbGEBXqkeUbSPherNnfLkQpy-pCGGgUiPDBn0haiubX5GS1bSkL9oIdpU_2LBu6NteMgLtDiBm_PMBVmTbjvwVgwH0ozQ1G-MGAVbBIRUtFrVJS-15paUNwKghwanPjLz9GKTifOhPQ3zAibPlu_hY_d-uzmPhZVblS1Yc3sf3ILCVgFzEyTVhKuNE-_fZvLO5IQBe4MQz7CSSclZ73RjMKYiUkqDIlztGJmgTRuq7JctgYcl4eIWj_pVBAFQoZLGIYedEwHxpYNJLaRgEK2PTAN-9Tjg572pXZFcm1uILgAnynvgNgMAoW453kDyd0crzt2_segTbCSrSD6JL-pKCF6jjLLBXSDfdbISFmip9HQ08g2KBxRSTEJOf4-SFBpnSlpWNMHGmlwmlCFH-fckLnFvTydxA0Xmqzo3NMVxoormCaJBbzosYmb3dnkPWcWge07Br5b7kvdBzDKcritawbJbuCrey5BohN_fzVNcH-MLselUnrf3nJl7XNl9QifaOLFXerjhqL-z3cMfeR5AKNhwcrR5S0X5VAR9deQf6xk_tzkrlIoXxcMauhvLbDN9qqeGfBJMSgLkQVMDRqzEbYpQ8qty0eGvYOto3ZlEtGreJrvQIAQu5QMfbhh3V4rT74YkV6hDLsMNlvW-sYnLqmQ6yacz_L9ESTrW3plu5l