local: use go1.13 UserConfigDir for InertiaDir

[bobheadxi]

🎟️ Ticket(s): for #611

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👷 Changes

• use the new UserConfigDir in the standard library to put global Inertia config in
• inertia remote config-path to show where stuff is going
• minor update to contributing docs to reflect changes

🔦 Testing Instructions

❯ ./inertia init --global
global Inertia configuration intialized at /Users/robertlin/Library/Application Support/inertia/inertia.global
❯ ./inertia remote config-path                                                   
global configuration directory: '/Users/robertlin/Library/Application Support/inertia'
global configuration path:      '/Users/robertlin/Library/Application Support/inertia/inertia.global'

[codecov]

Codecov Report

Merging #618 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #618   +/-   ##
=======================================
  Coverage   55.88%   55.88%           
=======================================
  Files          68       68           
  Lines        3377     3377           
=======================================
  Hits         1887     1887           
  Misses       1248     1248           
  Partials      242      242

Impacted Files	Coverage Δ
local/storage.go	26.16% <0%> (ø)	⬆️

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