New plugin: Network/bandwidth_primary.1s.sh

[fxlv]

I'd like to propose a new plugin that shows network bandwidth usage for primary interface in Megabits/s. It is based on bandwidth.1s.sh.

I thought about updating bandwidth.1s.sh but decided to make new one as there are too many changes.

What changes:

• only gather data about 1 interface (much faster and can now gather data for bit longer interval otherwise with 0.1 sec sampling rate the data is too spikey).
• gather data in Kbit/s and convert to display Mbits/s (perhaps this could be dynamic, based on current rate)

[tresni]

Closing and reopening to get latest travis configuration

[tresni]

@fxlv Please correct the shellcheck errors in the Travis CI build and we'll get this merged.

[fxlv]

@tresni thanks, fixed the shellcheck errors, waiting on Travis, they seem sot have some issues on their OSX build farm.