netstandard2.0 support

[mdmozibur]

A lot of project is still using netstandard2.0.
Targeting that will be beneficial to more developers.
Besides, not a lot code change is required.

I will submit a PR ASAP.

[DomCR]

Hi @mdmozibur,

If the change is minimal I won't have a problem to merge the PR, although I think is beneficial for other projects to update to higher versions of netstandard.

[mdmozibur]

@DomCR , create the pull request : here.
Just check it, only2 /3 lines of code changed.