Expose functions

[pan-pawel]

This PR is related to this issue #156

We would like to use source-map-explorer with e.g RAM Bundles (more info in issue)
To minimize the number of changes in the code we can just simply export a few of the functions that are used to calculate and present the data.
With those functions, we can go through the process of calculating and presenting ourselves

Changes

-export getExploreResult, writeHtmlToTempFile, loadSourceMap and computeFileSizes functions.

[coveralls]

Coverage remained the same at 96.552% when pulling 3170af6 on pan-pawel:expose-functions into fe52007 on danvk:master.