[bench] Automated benchmarking script (#1906)

[bench] Automated benchmarking script (#1906)

* Initial revised automated benchmarking script

* Updating nb_iterations and making loop infinite

* Allowing benchmarking params to be changed from cli

* Renaming old speed test

* Removing numpy dependency for cli

* Change filename and benchmakr on pr level

* Moving build outside loop and adding iterations param

* Moving benchmarking to seperate travis ci test

* Fixing typo and using unused variable

* Added mode labels and updated README accordingly

* Adding new mode 'current' that compraes facebook:dev against current hash

* Typo

* Reverting previous accidental diff

* Typo

* Adding frequency config variable to prevent github from blacklisting

* Added new argument for frequency of fetching new prs

* Updating documentation