explore promise api

[ctalkington]

by popular request and something ive wanted to try out. main concern was performance but maybe by moving newer node and such thats less of an issue. need to research options. open to suggestions

[ctalkington]

ref #207

[soyuka]

I think that performances may be improved on memory usage by removing processed entries from archiver._entries. Though, I find this array is a usefull when you want to compute progress (see #234).

Btw, not really related but WDYT about adding an event progress that would send the total bytes processed? With the pointer() it could also give live data about the compression ratio :D.
With this we can have progress and less memory usage!

Lmk your thoughts, I may be able to work on these.

ref #233

[ctalkington]

i do like the idea of a progress event over keeping so much in memory. this way someone can decide what to do with the progress data. lets give its own issue #244 for v2.1.0 most likely.

also #245 for compression ratio data.

[soyuka]

I closed this because I think it's resolved. Just thought that maybe you had other methods in mind that we could promisify?