Feature: Filter shortcuts in logs

[hami-sh]

This PR enables a set of shortcuts to be used when filtering through logs in k9s.

A new config file filters.yaml is defined, an example of which is shown below:

 filters:
   http_errors: '"status_code":[4-5]\d{2}'
   http_success: '"status_code":[2]\d{2}'
   info: 'info'

When using the / keybind in the logs view for a pod, a user can type the @ key followed by the name of the filter to access the pre-written regex or string they wish to match on. An example from the above would be @http_errors -- note how the title automatically resolves to the regex that was applied by the filter:

This is especially helpful when using complex regexes that a user might want to have close at hand when searching through logs.

[derailed]

@hami-sh Thank you for this PR!
I like the idea but I am not sure how useable this would be in several respects:

1. The naming is generic and does not reflect this customization only applied to logs
2. How does on fine tunes the filters ie if the log filters config file is changed how does one validate the exps?
3. No auto completion how can one recall what filters are available and what do they do? (especially in light of future self and/or an org sharing common regex based on their log layouts)

Also no docs or tests are present to ensure the behavior is properly advertised and correct.

[derailed]

Given these are only available in the logs we should name this accordingly aka LogFilters or something like that.

[derailed]

Not really useful best to fire off an error so the view can display a flash to the user.