Implement custom escaping rules

[josefpihrt]

fixes #47

[NameOfTheDragon]

I have implemented a custom escaper as attached below, and verified that this does meet my needs. If you merge this change, then I'll delete my own fork and nuget package and just use this one.

    public class NoDecimalEscaperProvider : IMarkdownEscaperProvider
    {
        public MarkdownCharEscaper GetDefaultEscaper() => new NoDecimalPointEscaper();
    }

    public class NoDecimalPointEscaper : MarkdownCharEscaper
    {
        public override bool ShouldBeEscaped(char value)
        {
            if (value == '.')
                return false;

            switch (value)
            {
                case '|':
                case '`':
                case '\\':
                case '*':
                case '_':
                case '{':
                case '}':
                case '[':
                case ']':
                case '(':
                case ')':
                case '#':
                case '+':
                case '-':
                case '!':
                case '>':
                case '~':
                case '=':
                case '<':
                case '^':
                case ':':
                case '"':
                    return true;
                default:
                    return false;
            }
        }
    }