Option to read file from an offset

[yuankunzhang]

Closes #38 and fixes some linting issues.

Added an --offset option to allow read file from a given offset. The behavior of the option is described as below:

• It supports both decimal and hexadecimal numbers.
• It doesn't support negative numbers.
• When it is larger than file size, it follows the current logic of handling an "empty" file: prints out a warning and exits. We should think about this more carefully.

[ndd7xv]

Hey again! Nice to see you around and thanks for the PR :smile:

I was testing out your changes and added some tweaks with the intent of improving the user features - most notably, these changes allow users to look at data earlier in the file even if they specify the --offset flag.

Let me know if you think there's anything wrong with what I've done, or if it breaks anything (I can ultimately revert or overhaul my changes if need be). If I get approval or don't hear back after a while, I'll go ahead and squash + merge!

[yuankunzhang]

@ndd7xv That's awesome!