Improve backtesting result formatting

[vertti]

tabulate has nice aligning of numbers but we were not using them because the columns contained % and BTC. I moved those to column headers. tabulate also has support for formatting so I could clean up the code a little.

Typical result now looks like:

====================== BACKTESTING REPORT ================================
pair         buy count    avg profit %    total profit BTC    avg duration
---------  -----------  --------------  ------------------  --------------
BTC_NAV            139            1.45          0.00601754            45.2
BTC_DGB            530           -0.09         -0.00154370            56.2
BTC_LSK            220           -0.25         -0.00164937            38.5
BTC_TRIG           157            1.12          0.00523099            44.3
BTC_XMR            229           -0.29         -0.00197289            45.5
BTC_XVG           1205            0.21          0.00728093            41.2
BTC_TRST           217           -0.46         -0.00302171            33.4
BTC_SWIFT          178           -1.52         -0.00813867            45.2
BTC_XEM            214           -0.48         -0.00299644            39.7
BTC_WAVES          214           -0.05         -0.00031410            31.5
BTC_ARK            236           -0.09         -0.00065559            32.2
BTC_XEL            121            1.70          0.00614261            31.1
BTC_SNT             78            0.44          0.00099623            58.9
BTC_PTOY           141            1.01          0.00424139            28.9
BTC_NXS            274           -0.45         -0.00370351            65.9
TOTAL             4153            0.05          0.00591371            43.4

[glonlas]

LGTM 👍