Correct stack traces using source-map-support

[tomchambers2]

Is it possible to use source-map-support to have stack traces refer to the uncompiled source? It's quite tricky seeing where errors in tests have come from at present.

[cmawhorter]

i've been adding ts-mocha -r source-map-support/register which gets stack traces mostly working.

but i've noticed some of the low-line-number (e.g. line 8) refer to something that i'm assuming typescript injects. the rest of the line numbers are accurate though so it hasn't been a problem to just ignore those