Error messages and more

This is the most solid release of Figwheel so far ... enjoy!!

Code context in Errors and Warnings

This means you will see the compile errors and warnings displayed
along with the offending code and a pointer to the position of the
failure. This is a fairly big improvement. I've found that even for
simple errors that I've become accustomed to, the new errors tend to
beam the information in much more quickly.

The code pointers are rough at times and no code context information
will displayed if Figwheel doesn't get any line and column information
from the compiler.

The display of these errors can still be improved further, and I plan
to do so. I just wanted to get the bits hooked up and a decent
display out the door first.

Please see the CHANGES.md file, there are a lot of improvements....