getContext feature implementation (similar to TraceKit context field)

[Ximik]

Description

A new method getContext is implemented. The method gets the surrounding source code for where exception occurs. Similar to TraceKit context, but much more clever and flexible.

I've seen 3 ways to implement this:

1. Create additional package.
2. Add the context field to the stackframe class.
3. Add the getContext method to the stacktrace-gps, which returns a separate object with the context source code.

I've decided to go the 3rd way, but if the stacktracejs team thinks, it should be done other way, just let me know.

Motivation and Context

It's very standard feature for all the error tracking tools, to show the surrounding source code (badly named "context") where the error occurs.
I've spent many hours fighting and hacking the TraceKit implementation (main problems: hard to configure the limits, very big and useless results for uglified code, inability to find out the line number of the code).

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• node_modules/.bin/jscs -c .jscsrc stacktrace-gps.js passes without errors
• npm test passes without errors
• I have read the contribution guidelines
• I have updated the documentation accordingly
• I have added tests to cover my changes