Search for hostnames in the `/etc/config/dhcp` file as well

[Kiougar]

Hey @pyrovski it's me again!

I think it would be great if wrtbwmon could also parse the /etc/config/dhcp file to search for hostnames. The great thing about this file is that when the user assigns a static IP from the DHCP settings in his router the following entry appears in the file:

config host
	option name 'BradleyDesktop'
	option dns '1'
	option mac '1c:1b:0d:e5:60:f1'
	option ip '192.168.1.69'
	option leasetime 'infinite'

also, when the user sets a custom hostname from the Hostnames page the following entry appears:

config domain
	option name 'TestDeviceName'
	option ip '192.168.1.167'

What do you think? Is this PR worthy?

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You can read the whole discussion that led to this solution in Kiougar/luci-wrtbwmon#13