compare getcontent to new value

[iricigor]

This is about #393

Content check is done before testing for -Force switch. This means these all three commands work fine, while only the first one updates file.

New-MarkdownHelp -Command Get-Date -OutputFolder MyFolder
New-MarkdownHelp -Command Get-Date -OutputFolder MyFolder -Verbose # file exists, no changes
New-MarkdownHelp -Command Get-Date -OutputFolder MyFolder -Verbose -Force

Second option is to move this check after -Force check (6 lines below), which would cause 2nd command to fail, but 3rd one would still work as expected.

[vors]

That's great, good job!