Add option to enable parsebib's `replace-tex` option

[malb]

This PR

• adds bibtex-completion-replace-tex which is passed to parsebib-read-entry for replace-tex
• adds helm-bibtex-ignore-diacritics which is passed to :diacritics in helm-source-bibtex

Both are nil by default, preserving the current behaviour of this package.

[malb]

The introduction of helm-bibtex-ignore-diacritics may be controversial given our previous conversation at #444.

With it set to nil I could not search for this entry:

@Article{JC:BLRSSS18,
  author =       "Shi Bai and
                  Tancr{\`e}de Lepoint and
                  Adeline {Roux-Langlois} and
                  Amin Sakzad and
                  Damien Stehl{\'e} and
                  Ron Steinfeld",
  title =        "Improved Security Proofs in Lattice-Based Cryptography: Using the {R}{\'e}nyi Divergence Rather than the Statistical Distance",
  pages =        "610--640",
  volume =       31,
  month =        apr,
  publisher =    "springer",
  year =         2018,
  journal =      "jcrypto",
  number =       2,
  doi =          "10.1007/s00145-017-9265-9",
}

in https://cryptobib.di.ens.fr/ using "stehle renyi"

With it set to t it works. TBH, I'm still confused about how this is meant to work in helm.

[malb]

Also, my use of cl-letf may or may not be controversial?

[tmalsburg]

Hi, I've been sick for some time and have a lot of stuff to deal with. I will look at this PR as soon as I find some time.