Git acceptance strategy

[Igmat]

It will be reference implementation for acceptance strategies (#68) and used instead of existing manual in standard preset (#60).

It should have two modes.

Strict (default)

1. When accept command used, commits stage (or everything if staged is empty) and replaces accepted field of baset config with current commit id.
2. When test command used baseliner's output should be immediately written to baseline file and compared to same file from git history at commit id that presents in accepted field of baset config.

Non-strict

1. When accept command used, commits stage (or everything if staged is empty).
2. When test command used baseliner's output should be immediately written to baseline file and compared to same file from git history at latest committed point.