Enhanced IFormattable/ToString Support

[tmilnthorp]

Following some discussion on #545:

It would be nice to extend the support for IFormattable and ToString.

From the other thread:

So for IFormattable we should make a list:

• "G" = current default implementation
• "A1", "A2"... "An" = abbreviation 1 or 2 or n in the list
• "A" = first abbreviation (equal to "A1")
• "Q" = quantity name (e.g. "Length")
• "V" value using default ToString() for double/decimal/etc.

var length = Length.FromMeters( 3.0 );
var str = length.ToString( "A2" ); 

For more complicated strings I think you'd want to specify on a more granular level. We don't want to parse all the number format types. For formatting the value as double, etc. you might want to do the following:

var length = Length.FromMeters( 3.0 );
string.Format( "{0,10:E3} [{1:A1}]", length.Value, length );

[angularsen]

Updated docs in String Formatting · angularsen/UnitsNet Wiki