save request results into sqlite database

[brandonsimpson21]

can now be invoked with

oha <url> --db-url loadtest.db

to store the vector of request results into a sqlite db.

considerations:

• table name: defaults to loadtest table to avoid another parameter
• there may be other data people may want to keep track of.

the table looks like this:

CREATE TABLE loadtest (
            url TEXT NOT NULL,
            duration REAL,
            status INTEGER,
            len_bytes INTEGER,
        )