add symbol for noeol

[shiluotang]

only show this symbol when g:WebDevIconsNoEndOfLineDefaultSymbol is configured

Requirements (please check off with 'x')

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

What does this Pull Request (PR) do?

show symbol for noeol status as the airline [!EOL]

How should this be manually tested?

when g:WebDevIconsNoEndOfLineDefaultSymbol is default configured

1. open a simple file in vim
2. change eof status by vim command ":set noendofline"
3. there should be no symbol for noeof on airline fileformat part

when g:WebDevIconsNoEndOfLineDefaultSymbol is configured by user

buffer verify

1. set g:WebDevIconsNoEndOfLineDefaultSymbol in vimrc
2. open a simple file in vim
3. change eof status by vim command ":set noendofline"
4. airline fileformat part should contain the value of "g:WebDevIconsNoEndOfLineDefaultSymbol"

file verify

1. set g:WebDevIconsNoEndOfLineDefaultSymbol in vimrc
2. create a simple file: echo "hello" > abc
3. open file in binary mode: vim -b abc
4. call vim command ":set noendofline", then save and exit
5. open file in normal mode: vim abc
6. airline fileformat part should contain the value of "g:WebDevIconsNoEndOfLineDefaultSymbol"

Any background context you can provide?

What are the relevant tickets (if any)?

Screenshots (if appropriate or helpful)