parse_config: improve config loading

[lasers]

This attempts config loading utf-8-sig first (rather than utf-8) with fallback to command file.

It looks like py3status switched from utf-8 to file (with fallback utf-8) sometimes in the past to better handle files. However, because file is now tried first, it introduced new issues such as...

1. make file a hard dependency.
2. bsd file doesn't have --mime-encoding
3. adding timeout to file subprocess call
4. slow

I'm suggesting that we try again with utf-8-sig first, then fallback to command file rather than staying with the other way around. The final fallback is default locale (instead of explicit utf-8), but we tried utf-8-sig first anyway.

---

Strategy Comparison.

case        | file first     | utf-8 first    | utf-8-sig first
------------+----------------+----------------+-----------------
ascii       | ok             | ok             | ok
utf-8       | ok             | ok             | ok
utf-8-sig   | parse_fail:bom | parse_fail:bom | ok
latin-1     | ok             | fallback->ok   | fallback->ok
utf-16      | ok             | fallback->ok   | fallback->ok

Benchmark.

branch                  | mean_seconds | vs_master
------------------------+--------------+----------------
master                  | 0.006011611  | baseline
parse_config_fallback   | 0.000251349  | 95.82% faster