ci: add markdown linter (#146)

spec/reactors/mempool/config.md

@@ -12,7 +12,7 @@ Environment: `TM_MEMPOOL_RECHECK=false`
 
 Config:
 
-```
+```toml
 [mempool]
 recheck = false
 ```

spec/reactors/mempool/functionality.md

@@ -35,7 +35,7 @@ What guarantees does it need from the ABCI app?
 
 The implementation within this library also implements a tx cache.
 This is so that signatures don't have to be reverified if the tx has
-already been seen before. 
+already been seen before.
 However, we only store valid txs in the cache, not invalid ones.
 This is because invalid txs could become good later.
 Txs that are included in a block aren't removed from the cache,

spec/reactors/mempool/reactor.md

@@ -7,7 +7,7 @@ See [this issue](https://github.com/tendermint/tendermint/issues/1503)
 Mempool maintains a cache of the last 10000 transactions to prevent
 replaying old transactions (plus transactions coming from other
 validators, who are continually exchanging transactions). Read [Replay
-Protection](https://github.com/tendermint/tendermint/blob/master/docs/app-dev/app-development.md#replay-protection)
+Protection](https://github.com/tendermint/tendermint/blob/8cdaa7f515a9d366bbc9f0aff2a263a1a6392ead/docs/app-dev/app-development.md#replay-protection)
 for details.
 
 Sending incorrectly encoded data or data exceeding `maxMsgSize` will result