Add excludedSymbols with reasonable defaults

[ghost]

No description provided.

[ghost]

History: Whilst scala.js testing I got some compile time errors as some macros are indeed being instrumented. This is not new - eg this shapeless class

Reason is that the current check just sees if a method returns an Expr but often a Tree is returned, as in that shapeless example

The reason I hit this in the scala.js version is that when it tries to to write data at compile time, it only has an sjs writer, that won't work at compile time as that is still in the jvm world

So this PR adds an option to specify a list of symbols to ignore, and defaults to Expr and Tree. This also means previous behaviour can be retained.

Potentially, the comment in the code "todo add way of allowing methods that return Expr" can be partially done by removing the excluded symbol at a project build level .Not perfect, but a step in the right direction.

Full usage documentation to appear in sbt-scoverage as that is where users will look.

[rorygraves]

This looks good to me - lets merge it - over to @gslowikowski / @sksamuel