Handling of comments in doctests

[mortenpi]

Julia Base has quite a few examples of potentially doctestable blocks with comments, e.g.:

# 32-bit system:
julia> UInt
UInt32

# 64-bit system:
julia> UInt
UInt64

They cannot be doctested because the comment is not part of the actual output, but perhaps we should figure out a convention allowing such blocks to be doctested as well, with the comments being automatically ignored.

[mortenpi]

Note: comment lines right after the REPL are already ignored, e.g.

julia> code()
# ignored comment
output

Ref: src/DocChecks.jl#L373-L400