Add a way to read commands from a file for send_command_set() about netmiko

[supertylerc]

I asked about this a few days ago. However, I think that there may be some things to consider. Creating an array of the files lines may be bad from a performance perspective. Chunking may be better, creating a smaller array, pushing the lines, and then going through the next X chunks. This is particularly true when the file contains entire configurations with comments.

This suggestion may fall under "premature optimization is the root of all evil", though. Take it with a grain of salt. :)

Also, as I require this feature for an upcoming project, I may implement it (crudely) and submit a PR with the change.

[ktbyers]

I think we should probably keep it simple...just read in the whole file and parse it into a list.

I don't see the performance of chunking as mattering.

The performance that possibly matter is how long it takes to send a whole bunch of configuration commands since send_config_set() calls send_command() which will by default delay 1 second per command.

[supertylerc]

I've written the code for this. I'll be submitting a PR later today or tomorrow.

[supertylerc]

This is in #59.