text: Title: Feature Request: SRT (Subtitle) Support
  Body: (I can't do this as a pull request, I'm not a Python programmer.)
  
  I would like to request that the translate file mode support SRT (Subtitle) files.
  The format of these is pretty simple, they are text files with an incrementing number, the next line has a time range in the form of `HH:MM:SS,HUN --> HH:MM:SS,HUN`, and finally the text which **may** include some HTML markup, primarily either `<b>` or` <i>`.  Sometimes there is a `<br>` (or `<br/>`).   A blank line separates text 'frames'.
  
  Mostly LibreTranslate needs to _ignore_ the number and the time ranges entirely, and for the text, treat it like HTML.
  There may already be Python code somewhere for handling SRT files.
  
  ###  Example:
  ```
  1
  00:00:02,830 --> 00:00:05,560
  <i>The miniature giant space hamster
  descends from the starry sky.</i>
  
  2
  00:00:05,700 --> 00:00:06,360
  Boo!  
  
  
  ```
  
  (If someone decides to work on this and needs sample SRTs, please contact me.)
  
  ### Use Case:
  Translating subtitle text is an obvious use of LibreTranslate.    For example, _Subtitle Edit_, an application to aid in editing or creating subtitle files, even has a "Google Translate" button with in it for machine translating lines of dialogue (it just opens a web browser).
  
  ### Notes:
  At present LibreTranslate handles srt (as txt) in kind of a mixed bag.   Sometimes it trims or strips out the time range and sometimes it changes the frame number although it's incredibly inconsistent on both.
  