Pass list of files via stdin for `--tui` mode

[rkaramc]

Is your feature request related to a problem? Please describe.
Running in --tui mode lists all files in current directory and its children. Would like to send a list of files to monitor to glow instead.

Describe the solution you'd like
Support a command like list-files | glow --tui --files
By default, stdin input will be considered to be a markdown file for preview. But --files flag will signal that the stdin input is a list of files (one per line).

Describe alternatives you've considered
No alternatives exist if I recall correctly.