Added the ability to run expressions in any command. Expressions can be

inserted in commands by using backticks:

(lldb) memory read `$rsp-16` `$rsp+16`
(lldb) memory read  -c `(int)strlen(argv[0])` `argv[0]`

The result of the expression will be inserted into the command as a sort of
preprocess stage where this gets done first. We might need to tweak where this
preprocess stage goes, but it is very functional already.

Added ansi color support to the Debugger::FormatPrompt() so you can use things
like "${ansi.fg.blue}" and "${ansi.bold}" many more. This helps in adding 
colors to your prompts without needing to know the ANSI color code strings.

llvm-svn: 141948