Add global default config

[sarjann]

Added the ability to specify your own default config. Think it's a nice convenience, in my case, I wanted to change the tmp directory to a hidden directory for example and it's a bit annoying to have to update the air.toml in each project.

[xiantang]

seems good but I i want to know is this the common case ?

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (b8f3808) 68.70% compared to head (129ab1b) 68.94%. Report is 5 commits behind head on master.

:exclamation: Current head 129ab1b differs from pull request most recent head d36e7d2. Consider uploading reports for the commit d36e7d2 to get more accurate results

Files	Patch %	Lines
runner/config.go	62.50%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #489 +/- ## ========================================== + Coverage 68.70% 68.94% +0.23% ========================================== Files 9 9 Lines 1061 1069 +8 ========================================== + Hits 729 737 +8 Misses 252 252 Partials 80 80 ```

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[sarjann]

seems good but I i want to know is this the common case ?

Hard to say without asking other people, but this doesn't break backwards compatibility and I think it's a nice feature. Also I think a lot of people will probably have their own defaults that they like.

[sarjann]

Not sure if there are any additional changes you want to make or if you're happy for this to be merged. @xiantang If you're fine with the changes I can add some tests (lines aren't currently covered), I'm having some trouble running tests locally but can figure it out.