Enable external formatters

[martin-schulze-vireso]

We have several requests for other formatters and it is hard to draw the line which to include into bats and which to redirect to simply consuming TAP. For further development, we should fix the set of existing formatters and enable easy integration for others. These are the necessary steps:

1. provide a (somewhat) stable interface (that should allow for testing if a formatter works). lib/bats-core/formatter.bash is a first step in that direction
2. allow for loading formatters from a custom location (add something like a BATS_FORMATTER_PATH)
3. Redirect existing PRs into an extra formatter project to bundle them there without holding up main development. This relates to following PRs: WIP: Add CppUnit XML as an output option #54 feat: Add lava support #300

• as a side note: Some formatters like the Lava formatter offer additional information ("measurements"), we need to find a way to incorporate these in a way that at least avoids tripping up other formatters and at best making it usable across the board.

[martin-schulze-vireso]

As an initial increment in this direction, we should allow for specifying absolute paths: If the -F/--formatter argument has a leading / we just assume this is the file meant to be executed.

[martin-schulze-vireso]

A partial solution has been implemented in #602. The rest needs to be reevaluated.