v0.8.6

1. Upgrade mintty to 2.8.1.
2. Add 6-set-default-shell.bat (usage).

If your wsl-terminal version number is higher than 0.8.0, can upgrade it with ./cmdtool update (usage).

sha256sum	filename
c21af7d3a15711a8a863cbd46b092acf42dc9b6456d9cfb1a3605ef1696ced52	wsl-terminal-0.8.6.7z
17b6273d1aa26a3c13d5862bbb3afbfde19f0afe3bedf1a584c8c50869526b4a	wsl-terminal-0.8.6.zip