zsh-completion: fix issue with [.md, add basic -L completion, remove extra _files args

[cho-m]

Changes information:

• Use array commands=(...) and _describe to deal with '[.md' & empty cache scenario. Fixes #166
• Hide tldr --list stderr (2>/dev/null) which breaks completion with empty cache
• Remove sed since #112 changed commas to newlines
• Add new sed-equivalent replacement (: -> \:) using native ZSH ${name//pattern/repl} since colon is special character in ZSH completions
• Add basic support for -L, --language flag from #125. In future, can consider adding extra completions maybe based on caches pages.{lang} folders.
• Remove extraneous completion of file names for positional arguments (i.e. '*:file:_files')

The following are output from ZSH 5.8 with locally built master branch.

---

Some errors shown with original completions:

1. [.md:

   

2. Empty cache stderr:

   

---

Output of ZSH with new completions

1. No issue with [.md:

   

2. Empty cache, no completions shown (ignore dimmed font due to zsh-autosuggestions plugin):

   

3. Empty cache, flag completions after -:

   

[dbrgn]

Thanks @cho-m for the fix and @niklasmohrin for the review/test!