Fix referencing isSilent under strict mode

[MasterOdin]

Fixes db-migrate/node-db-migrate#590

This PR fixes a bug where when running db-migrate under strict mode (such as via its programmable api in a TS script), you'd get an undefined error when trying to reference this.isSilent. This is because under script mode, the this object is made undefined in a function as opposed to referencing the global scope (see "No this substitution" section in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode#changes_in_strict_mode). Instead, we make the isSilent variable accessible on the exports object which gives the same overall characteristics as the global this (such as being accessible and changeable via require), but that exports.isSilent can be safely referenced within the functions.

An alternative approach could be:

this.isSilent = false;
var that = this;

// use that.isSilent everywhere

but this seemed more complicated than using exports.isSilent.