Debugger Improvements

[zer0x64]

• Support empty lines as the previous command, like GDB does. The most common use case would be to quickly step through the code and to dump data with x.
• Support more syntax related to the x command. This can be quite tricky and might require the use of peg, which I already used on another project and know how to use.
• Support command line history. Note that this is something that might be impossible to support portably