Add custom columns to output

[0x0f0f0f]

Idea to add custom columns to the tables:

@timeit could take an additional NamedTuple that refers to custom, user specified columns.
addcolumn! can be used to insert a new column in a TimerOutput.
setaccumulator! could be used to specify which function is used to accumulate the custom value in a custom column. Default can be +.

to = TimerOutput()
addcolumn!(to, :foo, 0) # last arg is default value for new entries
setaccumulator!(to, :foo, (+)) # function f to compute accumulated = f(accumulated, new_value) 
addcolumn!(to, :bar, 1)
setaccumulator!(to, :bar, (*))

@timeit to "nest 1" begin
    sleep(0.1)
    @timeit to "level 2.1" sleep(0.1) (foo=1, bar=2)
    for i in 1:20; @timeit to "level 2.2" sleep(0.02) (,bar=2); end
end
@timeit to "nest 2" begin
    for i in 1:30; @timeit to "level 2.1" sleep(0.01); end
    @timeit to "level 2.2" sleep(0.1) (foo=4.5, bar=3)
end

Inside of begin ... end blocks, another macro @accum can be used in order to accumulate values computed inside a timed block.

function f(x) = (sleep(rand(1:x)); rand(1:x))

addcolumn!(to, :randn, 0)
@timeit to "rand num" begin 
    n = f(10)
    @accum to (,randn=n) # should not be timed
end