Deprecate --watch and --ext flags in favour of stdin arguments

[maaslalani]

Is your feature request related to a problem? Please describe.
Deprecate --watch and --ext flags in favour of stdin arguments.
Since you can use stdin to get a list of filenames the current flags are redundant because --watch specifies a directory and --ext specifies the extension. Using stdin can always replace these in a more unix way.

For example:

1. Watching all markdown files and running glow on README.md

crow -e .md glow README.md

fd .md | crow glow README.md

2. Watching all go files in the src directory and rerunning tests

crow -w src -e go go test ./...

fd src -e go | crow go test ./...