[wt.exe] -H and -V parameters shouldn't be case-sensitive

# Environment

```none
Microsoft Windows [Version 10.0.18363.535]
```

# Steps to reproduce
```
PS C:\> wt `; split-pane -h cmd.exe
PS C:\> wt `; split-pane -v cmd.exe
```
# Expected behavior

Commands run without a problem when I specify -h and/or -v.

# Actual behavior

Help dialog pops-up complaining about the wrong syntax. Btw, help dialog is not the same for -v and -h.
