REPL help for LaTeX tab completion

[stevengj]

In this PR, typing a Unicode symbol into the REPL help now prints information about how to type it using the LaTeX tab completion, if possible:

help?> √
"√" can be typed by \sqrt<tab>

search: √

  sqrt(x)

  Return \sqrt{x}. Throws DomainError for negative Real arguments. Use complex
  negative arguments instead. The prefix operator √ is equivalent to sqrt.

help?> x̂₂
"x̂₂" can be typed by x\hat<tab>\_2<tab>

search:

Couldn't find x̂₂
Perhaps you meant xor, exit, exp, exp2, expm, Expr, hex, max, !, !=, !==, $, % or &
  No documentation found.

  Binding x̂₂ does not exist.

While I was editing this code, I also made it possible to get the help text in a user-supplied io stream rather than STDOUT, which will help with IJulia and similar enviroments.

[MichaelHatherly]

LGTM, I assume that the usefulness of this feature is more for longer tab-completed names such as

help?> x_φ̄⁵
"x_φ̄⁵" can be typed by x_\varphi<tab>\bar<tab>\^5<tab>

that could be tab completed with x_<tab> rather than single symbols such as

help?> φ
"φ" can be typed by \varphi<tab>

where the user would have had to input the symbol with \varphi<tab> to begin with anyway (usually, though perhaps they copy-pasted it from some where).

[MichaelHatherly]

Could use two definitions for dispatch to avoid needing the conditional: macro repl(ex) ... and macro repl(io, ex) ..., not too important though really.

[stevengj]

Oh, I didn't realize that dispatch could be used for macros. Learn something new every day.

[stevengj]

@MichaelHatherly, I think it is useful for single symbols, too. There are other (more annoying) ways to type these symbols than by tab completion.

For example, you could be looking at someone else's code, see a symbol you don't know how to type, and find out by copy-and-pasting it into the REPL help.

Or you could type the symbol on MacOS by the character palette, or by copying and pasting from a web page, or ...

[stevengj]

(For example, if you look at ?sqrt, the help text tells you about √ but not how to type it, so you could copy-and-paste that into ?√.)

[stevengj]

Should be good to merge; the Travis failure is the unrelated x86 problem we are having nowadays.