Support for saving only part of the logs

[kasia-de]

Allow highlighting only a section to be removed from the capture, or for only that section to be saved.

1. I want to capture only up to a certain point, excluding the last X lines.

2. I want to capture a crash and a certain surrounding area

These would compliment #14

[kasia-de]

To clarify, what I had in mind was a vertical slider, and the highlighling would be similar to how Github highlights selected lines.