Added more formatting attributes for -D

[keithbowes]

I was noticing that you can now use --color/-D to set colors, which is so much nicer than the LESS_TERMCAP_ environment variables. However, there was something missing; with the environment variables, I make blinking text bold instead of blinking, but that didn't seem to be possible with -D, so I added it.

The new symbols * and _ for bold and underline are based on common convention/Markdown. ~ for standout comes from the C not bitwise operator (since terminals tend to reverse the colors in standout mode; I originally made it \ but I thought that might get confused with an escape character; some Markdown dialects use ~ for strikethrough, so ~ might not be the best choice either). & for blink is arbitrary.

This PR however does have one limitation: It still only works for lowercase letters. It's still not possible to make the prompt bold, for example. I might look into why later.

[gwsw]

Thanks. In 608c8e5 I have implemented this differently, so these new attribute characters are supported for both uppercase and lowercase text types.