Add static IP command line switch

[imcsk8]

There are use cases in which a container needs a persistent static IP.
This patch adds a switch to set a static IP for a container.

Eg.

"hooks": {
"prestart": [
{
"path": "/home/example/go/src/github.com/genuinetools/netns/netns",
"args": ["netns", "--static-ip", "172.19.0.2"]
}
]
}

closes #11

[imcsk8]

PTAL @jessfraz

[codecov]

Codecov Report

Merging #12 into master will decrease coverage by 0.15%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #12      +/-   ##
=========================================
- Coverage   10.66%   10.5%   -0.16%     
=========================================
  Files           5       5              
  Lines         272     276       +4     
=========================================
  Hits           29      29              
- Misses        241     245       +4     
  Partials        2       2

Impacted Files	Coverage Δ
network/create.go	0% <0%> (ø)	⬆️

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[jessfraz]

thanks!