exp: zapslog / replaceattr support

[alok87]

Is your feature request related to a problem? Please describe.
Its a request for extending zapslog

Describe the solution you'd like
Extend to support using packages for masking centrally
so that using packages like this becomes possible https://github.com/m-mizutani/masq

[alok87]

replaceAttr injection helps

// apply replaceAttr if defined (this will mask)
		if h.replaceAttr != nil {
			attr = h.replaceAttr(h.groups, attr)
		}