Version 2.0.0 Beta 2

This is a beta version because there are still features and fixes
planned for v2.0.0. This version should be as stable as v1.12.

There are mostly bug fixes of bugs in v2.0.0-beta.1.
There is one new data source.

- cpu: Internal function `__lp_cpu_count()` to count CPU cores (e5047c0)
- dirstack: Directory stack data source (#625, f35d9ac)
- tests: Tests for `_lp_battery()`/`pmset` (c0e74b8, 9e205f5)
- tests: Tests for `_lp_connection()`/`who`+`ps` (23eb3f2)
- tests: Tests for `_lp_detached_sessions()`/`screen`+`tmux` (23eb3f2)
- tests: Tests for `__lp_temp_sensors()`/`sensors` (23eb3f2)
- tools: An external tool tester (b699dea, 9a00ead)

- tests: Make test suite not exit on first failing test (44e3a6f)
- tests: Make test suite use `set -u` (1fe1559)

- battery/MacOS: Non-quoted `$IFS` set command (#638, 4ebc26e)
- docs: Typos and confusing wording (3c0846a, 298693e)
- sensors: Redirect sensors error output to `/dev/null` (#445, 4a52696)
- theme: Non-usage of `$POWERLINE_PYTHON_ENV_MARKER` (#636, e908ec8)
- zsh: Zsh special variables (#634, 6f1dbcf)

- tests: `pmset-simulator` file, now integrated into `pmset` tests
  above (c0e74b8)