Add DB option enable to select other DB

[disktnk]

This PR is for supporting other DB, such as MySQL or PostgreSQL

• split DB_* to chainerui.db
  • DB engine and session are set in chainerui.db
• add --db option, ex: chainerui --db <db-url> server
  • if --db is empty, use SQLite
• fix miner bug
  • wrong table name
• refactoring test_app.py, simplified

I'll sent another PR

• document
• add test using other DB

[disktnk]

Test environment memo:

• Docker version 17.12.1-ce, build 7390fc6

PostgreSQL

• Postgres 10.5
• psycopg2-binary 2.7.5
• URL example: postgresql://user:pass@localhost:5432

MySQL

• MySQL 8.012
• mysqlclient 1.3.8
• URL example: mysql+mysqldb://user:pass@127.0.0.1:3306/chainerui

[makky3939]

LGTM

I tried this p-r with postgres, mysql and sqlite(specified).