[feature] Support filter sg run results by metavar regular expression

[HerringtonDarkholme]

Related: #378

We can only use one single pattern to search code for command line sg run. This is convenient but less powerful to sg scan.
For example, if users want to find userCol.update or itemCol.update, a single pattern $COL.update cannot be done on command line. The work around is to write a temporary rule in YAML and use sg scan -r temp.yml to scan code base.

We can build a better CLI option to accept regular expression for meta-variables on command line.

[HerringtonDarkholme]

I have an idea like this:
sg run will have a new option -f --filter to filter match results based on the metavariable.

The filter will have three forms:

• -f META_VAR=pattern will match the meta-variable against ast-grep pattern pattern.
• -f META_VAR/regexp/ will match the meta-variable against regular expression regexp.
• -f META_VAR:kind_id will match the meta-varaible with nodes of the syntax kind kind.

This is a mini-syntax to keep in sync with constraints in the YAML format. Maybe it is too hard to learn and even harder to search on web.

Please leave comments here if you have any feedback! It can be a simple "love it"/"hate it". Or a lengthy paragraph explaining your preference. Of course I welcome your proposal!

[HerringtonDarkholme]

Alternative design:

• --filter-pattern META_VAR=pattern
• --filter-regex META_VAR=regex
• --filter-kind META_VAR=kind

which one is better?