add symbol for byte order marker (utf-8)

[Snaptags]

Requirements (please check off with 'x')

• I have read the Contributing Guidelines
• I have read or at least glanced at the FAQ
• I have read or at least glanced at the Wiki

What does this Pull Request (PR) do?

Add the possibility to define a symbol to be displayed IF an utf-8 file contains a byte order marker (BOM)

How should this be manually tested?

• add
  let g:WebDevIconsUnicodeByteOrderMarkerDefaultSymbol = 'X'
  to your vimrc

• open any utf-8 file. If it has a bom, the X will show. If not:
  :set bomb

Any background context you can provide?

What are the relevant tickets (if any)?

Screenshots (if appropriate or helpful)

The symbol used here is  /uE287

[ryanoasis]

@Snaptags Hey thanks for this! I will try to get to it this weekend (maybe)