Add graph visitor that ignores null values

[gabriel-samfira]

When serializing to yaml, a new -Options OmitNullValues options will skip any mapping that has a null value.

Example:

PS /home/gabriel/powershell-yaml> import-module ./powershell-yaml.psd1
PS /home/gabriel/powershell-yaml> $testObj = @{"a"=1; "b"=$null; "c"="aa"}
PS /home/gabriel/powershell-yaml> ConvertTo-Yaml $testObj                                                                                  
b: 
a: 1
c: aa

PS /home/gabriel/powershell-yaml> ConvertTo-Yaml $testObj -Options OmitNullValues
a: 1
c: aa

PS /home/gabriel/powershell-yaml> 

Fixes: #129