Add support for parameterized statements and nullable types

[sgalsaleh]

Changes / Notes:

• Support for parameterized statements (e.g. parameter binding)
• Support for nullable types, similar to the database/sql package (e.g. sql.NullString, sql.NullInt64, etc...)
• Tests for the above added functionalities
• Other minor improvements
• Backwards compatible, no need for a v2

Examples:

• WriteParameterized:

  wr, err := conn.WriteParameterized(
  []gorqlite.ParameterizedStatement{
      {
          Query:     "INSERT INTO secret_agents(id, name, secret) VALUES(?, ?, ?)",
          Arguments: []interface{}{7, "James Bond", "not-a-secret"},
      },
  },
  )

• WriteOneParameterized:

  wr, err := conn.WriteOneParameterized(
  gorqlite.ParameterizedStatement{
      Query:     "INSERT INTO secret_agents(id, name, secret) VALUES(?, ?, ?)",
      Arguments: []interface{}{7, "James Bond", "not-a-secret"},
  },
  )

• QueryParameterized / QueueParameterized: same syntax as WriteParameterized

• QueryOneParameterized / QueueOneParameterized: same syntax as WriteOneParameterized

• Using nullable types:

  var name gorqlite.NullString
  rows, err := conn.QueryOne("select name from secret_agents where id = 7")
  for rows.Next() {
  err := rows.Scan(&name)
  }
  if name.Valid {
  // use name.String
  } else {
  // NULL value
  }

[sgalsaleh]

@otoolep This PR is ready for review. Could you please review and let me know if I should make any changes?

[otoolep]

Thank your for your contribution - looks promising, I will merge once I understand the point of the Valid flags.

[sgalsaleh]

@otoolep Thanks for the quick review. I've just added an example for using nullable types to the README. Let me know if it's still unclear.

[otoolep]

Great -- thank you for this.