Added context sources line to error message

[eGust]

this PR is similar to #159

1. Option error_context_lines can be overridden by ERROR_CONTEXT_LINES
2. Added 3 test cases
3. Updated README
4. Follows rubocop rules.

[eGust]

Seems there is nothing I can do about the CI failure:

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
10.47s$ rvm use 1.9.3 --install --binary --fuzzy
curl: (22) The requested URL returned error: 404 Not Found
Required ruby-1.9.3 is not installed - installing.
curl: (22) The requested URL returned error: 404 Not Found
Searching for binary rubies, this might take some time.
Requested binary installation but no rubies are available to download, consider skipping --binary flag.
Gemset '' does not exist, 'rvm ruby-1.9.3 do rvm gemset create ' first, or append '--create'.
The command "rvm use 1.9.3 --install --binary --fuzzy" failed and exited with 2 during .
Your build has been stopped.