permit callsigns that begin with a number

[swharden]

Portion of an email from Ross:

you program spits out calls starting with a numeral of which there are many countries that have first Char as a numeral

e.g., 9A9UN and 6O0O

The relevant line of code is here:

WsprSharp/src/WsprSharp/Encode.cs

Lines 94 to 95 in 328849b

	if (!char.IsLetter(callsign.First()))
	throw new ArgumentException("callsign must start with a letter");

This issue will track removing this constraint to allow callsigns that begin with a number