basenc: add help texts for options

[cakebaker]

This PR adds some missing help texts.

Before:

Options:
  -d, --decode          decode data
  -i, --ignore-garbage  when decoding, ignore non-alphabetic characters
  -w, --wrap <COLS>     wrap encoded lines after COLS character (default 76, 0 to disable wrapping)
      --base64          
      --base64url       
      --base32          
      --base32hex       
      --base16          
      --base2lsbf       
      --base2msbf       
      --z85             
  -h, --help            Print help
  -V, --version         Print version

After:

Options:
  -d, --decode          decode data
  -i, --ignore-garbage  when decoding, ignore non-alphabetic characters
  -w, --wrap <COLS>     wrap encoded lines after COLS character (default 76, 0 to disable wrapping)
      --base64          same as 'base64' program
      --base64url       file- and url-safe base64
      --base32          same as 'base32' program
      --base32hex       extended hex alphabet base32 https://google.com
      --base16          hex encoding
      --base2lsbf       bit string with least significant bit (lsb) first
      --base2msbf       bit string with most significant bit (msb) first
      --z85             ascii85-like encoding;
                        when encoding, input length must be a multiple of 4;
                        when decoding, input length must be a multiple of 5
  -h, --help            Print help
  -V, --version         Print version