Add a possibility to attach files

[0x111]

Add a way to attach files.

Now you can attach one or more files (restrictions apply based on paste.me limits) to the paste while creating it.

Example usage:

pasteme-cli --name "My paste" --body "License file attached" --file ./LICENSE

If you would like to attach one or more files, you can call --file multiple times like this:

pasteme-cli --name "My paste" --body "License file attached" --file ./LICENSE --file README.md

[codecov]

Codecov Report

Merging #1 into master will decrease coverage by 5.04%.
The diff coverage is 84.00%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
- Coverage   88.28%   83.24%   -5.05%     
==========================================
  Files           1        1              
  Lines         128      179      +51     
==========================================
+ Hits          113      149      +36     
- Misses         10       19       +9     
- Partials        5       11       +6     

Impacted Files	Coverage Δ
pasteme_cli.go	83.24% <84.00%> (ø)

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[0x111]

Test coverage is fair enough, it does cover all major functions including the uploading of files is being tested too. So I think the coverage is good at the moment that is why I will accept this as-is.