Make languages & policies user-configurable

[agriffis]

Add three configurables:

• g:vue_indent_languages replaces s:languages. It can be configured
  before or after loading indent/vue.vim, and can be modified on the
  fly.

• g:vue_indent_open_close_tags configures the policy for indenting
  open/close tags. It defaults to 0 which forces open/close tags for
  sections in a vue single-file component to the first column.

• g:vue_indent_first_line configures the policy for indenting the
  first line after an open tag. It defaults to -1 which uses the
  autoindent default, i.e. the same column as the open tag. It will
  also preserve existing indentation when reindenting, which means
  that authors can use their preferred first line indentation without
  needing any special configuration.

Additionally g:vue_indent_first_line can be overridden per-language by
specifying a value for language.first_line. We use this for the html
language configuration to defer to html.vim for indenting the first line
of an html template.

Minor changes:

• Since s:get_indentexpr is called lazily, it resets indentexpr before
  returning.

• Use searchpair() to find the closing tag instead of a hard-coded list.

• For development and debugging, it's possible to :echo GetVueIndent()
  which also enables error reporting and messages that are normally
  squelched when called as indentexpr. Note that although GetVueIndent()
  uses getpos('.') properly if line number isn't supplied, language
  indenters vary between getpos('.') and v:lnum, so this is mostly
  useful for debugging vue.vim's internal operation.

[agriffis]

By the way, I think it would make a lot of sense for g:vue_indent_languages to eventually become g:vue_languages with a common structure for configuring indent/syntax simultaneously. I guess that raises the question of whether it's better to do that rather than merging this directly. Let me know what you think.

[adriaanzon]

Thanks for the effort you have put into this!

I took a quick look at the changes, and left a comment at a line that caught my eye. I will have a further look at the changes in this PR sometime later.

[adriaanzon]

You could use is# here, which also checks if they are the same instance/type.

I think I will eventually s/==/is/g the entire codebase.