fix: return str instead of list when executing tldr -l

[uunnxx]

Return str separated by \n.

It helps when using by other programs, like fzf
I've alias:
alias ch="tldr \$(tldr -l | fzf)"
so it'll search through all pages.

Closes #221.

[acuteenvy]

This breaks the --search option. All you need to do to output the list as a newline-separated string is to change

print(get_commands(options.platform))

to

print('\n'.join(get_commands(options.platform)))

on line 507.

[vitorhcl]

@kbdharun @acuteenvy as this is a minor change, I think you can apply @acuteenvy's suggestion and merge this

[kbdharun]

LGTM, Thanks for your contribution.